For Peer Review Only - IUPAC

For Peer Review OnlyGlossary of Physical Organic Chemistry

Journal: Pure and Applied Chemistry

Manuscript ID PAC-REC-18-10-10.R6

Manuscript Type: Recommendation

Date Submitted by the Author: 20-Apr-2021

Complete List of Authors: Perrin, Charles; University of California, San Diego, Department of Chemistry & BiochemistryAgranat, Israel; Hebrew University of JerusalemBagno, Alessandro; University of Padova Faculty of Mathematics Physics and Natural SciencesBraslavsky, Silvia; Max Planck Institute for chemical Energy Conversion, Fernandes, Pedro; University of PortoGal, Jean-François; Université de Nice Sophia Antipolis UFR SciencesLloyd-Jones, Guy; University of Edinburgh, ChemistryMayr, H.; Ludwig-Maximilians-Universität München, Department für Chemie und Biochemie; Murdoch, Joseph; Dupont Central Research and DevelopmentNudelman, Norma; University of Buenos AiresRadom, Leo; University of Sydney, School of ChemistryRappoport, Zvi; The Hebrew University, Organic Chemistry; Ruasse, Marie-Françoise; ITODYSSiehl, Hans-Ullrich; Universitat Ulm Fakultat fur Naturwissenschaften, Institute of Organic Chemistry ITidwell, Thomas T.; University of Toronto, Department of ChemistryUggerud, Einar; Universitetet i Oslo, Kjemisk instituttWilliams, Ian; University of Bath , Department of Chemistry

Keywords:acidity, aromaticity, catalysis, chemical equilibrium, hydrogen bonding, kinetics, mechanism, nomenclature, organic chemistry, reaction mechanisms, solvation, structure-reactivity

Author-Supplied Keywords:

P.O. 13757, Research Triangle Park, NC (919) 485-8700

IUPAC

For Peer Review Only

1 GLOSSARY OF TERMS USED IN2 PHYSICAL ORGANIC CHEMISTRY345 Abstract. This Glossary contains definitions, explanatory notes, and sources for terms 6 used in physical organic chemistry. Its aim is to provide guidance on the terminology of 7 physical organic chemistry, with a view to achieving a consensus on the meaning and 8 applicability of useful terms and the abandonment of unsatisfactory ones. Owing to the 9 substantial progress in the field, this 2021 revision of the Glossary is much expanded

10 relative to the previous edition, and it includes terms from cognate fields. 111213 INTRODUCTION TO THE 2021 REVISION14 General Remarks1516 The first Glossary of Terms Used In Physical Organic Chemistry was published 17 in provisional form in 1979 [1] and in revised form in 1983, incorporating modifications 18 agreed to by IUPAC Commission III.2 (Physical Organic Chemistry) [2].19 A further revision was undertaken under the chairmanship of Paul Müller, which 20 was published in 1994 [3]. The work was coordinated with that of other Commissions 21 within the Division of Organic Chemistry. In 1999 Gerard P. Moss, with the assistance 22 of Charles L. Perrin, converted this glossary to a World Wide Web version [4]. The 23 Compendium of Chemical Terminology [5] (Gold Book) incorporated many of the terms 24 in the later version.

25 This Glossary has now been thoroughly revised and updated, to be made 26 available as a Web document. The general criterion adopted for the inclusion of a term 27 in this Glossary has been its wide use in the present or past literature of physical-28 organic chemistry and related fields, with particular attention to those terms that have 29 been ambiguous. It is expected that the terms in this Glossary will be incorporated within 30 the on-line version of the IUPAC Gold Book, which is the merged compendium of all 31 glossaries [5].

32 The aim of this Glossary is to provide guidance on the terminology of physical-33 organic chemistry, with a view to achieving a far-reaching consensus on the definitions 34 of useful terms and the abandonment of unsatisfactory ones. According to Antoine 35 Lavoisier "Comme ce sont les mots qui conservent les idées et qui les transmettent, il 36 en résulte qu'on ne peut perfectionner le langage sans perfectionner la science, ni la 37 science sans le langage,“ (As it is the words that preserve the ideas and convey them, 38 it follows that one cannot improve the language without improving science, nor improve

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1 science without improving the language.”) [6]. Our approach has been to take or update 2 entries from the previous glossary, whereas new terms were added by virtue of their 3 usage in the current literature and the diverse knowledge of the members of the Task 4 Force.

5 The Task Force is pleased to acknowledge the generous contributions of many 6 scientists who helped by proposing or defining new terms or by criticizing or modifying 7 existing ones. The following members of the Task Force have contributed to this 8 revision:9

10 Israel Agranat Charles L. Perrin, Chair11 Alessandro Bagno† Leo Radom12 Silvia E. Braslavsky Zvi Rappoport13 Pedro A. Fernandes Marie-Françoise Ruasse†

14 Jean-François Gal Hans-Ullrich Siehl15 Guy C. Lloyd-Jones Yoshito Takeuchi16 Herbert Mayr Thomas T. Tidwell17 Joseph R. Murdoch Einar Uggerud18 Norma Sbarbati Nudelman Ian H. Williams1920 († = deceased) 21 This version was prepared for publication by Charles L. Perrin, Silvia E. Braslavsky, 22 and Ian Williams. We especially note the able technical assistance of Gerard P. Moss 23 (Queen Mary University of London) and the advice of Christian Reichardt (Marburg). 24 We also thank the several reviewers whose comments improved the presentation, 25 especially Jan Kaiser (University of East Anglia).2627 Arrangement, Abbreviations, and Symbols28 The arrangement is alphabetical, terms with Greek letters following those in the 29 corresponding Latin ones. Italicized words in the body of a definition, as well as those 30 cited at the end, point to relevant cross-references. Literature references direct the 31 reader either to the original literature where the term was originally defined or to 32 pertinent references where it is used, including other IUPAC glossaries, where 33 definitions may differ from those here.34 Definitions of techniques not directly used for measurements in physical-organic 35 chemistry are not included here but may be consulted in specialized IUPAC texts, 36 including the IUPAC Recommendations for Mass Spectrometry [7], the Glossary of 37 Terms Used In Theoretical Organic Chemistry [8], the Glossary of Terms Used in 38 Photochemistry 3rd edition [9], the Glossary of Terms Used in Photocatalysis and 39 Radiation Catalysis [10] and the Basic Terminology of Stereochemistry [11].

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1 In accordance with IUPAC recommendations [12] the symbol ‡ to indicate 2 transition state ("double dagger") is used as a prefix to the appropriate quantities, e.g., 3 ∆‡G rather than ∆G‡. In equations including a logarithmic function the procedure 4 recommended by IUPAC was adopted, i.e., to divide each dimensioned quantity by its 5 units. Since this procedure often introduces a cluttering of the equations, we have in 6 some cases chosen a short-hand notation, such as ln {k(T)}, where the curly brackets 7 indicate an argument of dimension one, corresponding to k(T)/[k(T)], where the square 8 brackets indicate that the quantity is divided by its units, as recommended in [12].9

10 Note on the identification of new and/or revised terms.11 Terms that are found in the previous version of this Glossary [3,4] and currently 12 incorporated in the IUPAC “Gold Book” [5] are identified with a reference to the Glossary 13 of Terms used in Physical Organic Chemistry (1994), i.e., [3], whereas revised terms 14 are designated as rev[3]. Minor changes such as better wording, additional cross-15 referencing, or reorganization of the text without changing the concept are, in general, 16 not considered revisions. However, the improved version should replace the older one 17 in the “Gold Book”. New terms and terms from other IUPAC documents are not 18 identified as such. In many cases new references have been added in the definitions.192021

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1 A factor2 Arrhenius factor3 (SI unit same as rate constant: s–1 for first-order reaction).4 Pre-exponential factor in the Arrhenius equation for the temperature dependence of a 5 reaction rate constant.6 Note 1: According to collision theory, A is the frequency of collisions with the correct 7 orientation for reaction.8 Note 2: The common unit of A for second-order reactions is dm3 mol-1 s-1.9 See also A value, energy of activation, entropy of activation.

10 See [12,13].11 rev[3]1213 A value14 Steric substituent parameter expressing the conformational preference of an equatorial 15 substituent relative to an axial one in a monosubstituted cyclohexane.16 Note 1: This parameter equals rGº for the equatorial to axial equilibration, 17 in kJ mol–1. For example, ACH3 is 7.28 kJ mol–1, a positive value because an axial 18 methyl group is destabilized by a steric effect.19 Note 2: The values are also known as Winstein-Holness A values.20 See [3,14,15]. 2122 abstraction23 Chemical reaction or transformation, the main feature of which is the bimolecular 24 removal of an atom (neutral or charged) from a molecular entity.25 Examples:26 CH3COCH3 + (i-C3H7)2N– → [CH3COCH2]– + (i-C3H7)2NH27 (hydron abstraction from acetone)28 CH4 + Cl· → CH3· + HCl 29 (hydrogen atom abstraction from methane)30 See detachment.31 [3]3233 acceptor parameter (A)34 acceptor number (deprecated). Unit and dimension 1.35 Quantitative measure, devised by Gutmann [16] of the Lewis acidity of a solvent, based 36 on the 31P chemical shift of dissolved triethylphosphine oxide (triethylphosphane oxide).37 Note: The term acceptor number, designated by AN, is a misnomer and ought to be 38 called acceptor parameter, A, because it is an experimental value [17].39 rev[3]40

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1 acid2 Molecular entity or chemical species capable of donating a hydron (proton) or capable 3 of forming a bond with the electron pair of a Lewis base.4 See Brønsted acid, Lewis acid, Lewis base. 5 See also hard acid.6 [3]78 acidity9 (1) Of a compound:

10 Tendency of a Brønsted acid to act as a hydron (proton) donor, or tendency of a Lewis 11 acid to form Lewis adducts and -adducts.12 Note: Acidity can be quantitatively expressed by the acid dissociation constant of the 13 compound in water or in some other specified medium, by the association constants for 14 formation of Lewis adducts and -adducts, or by the enthalpy or Gibbs energy of 15 deprotonation in the gas phase.16 (2) Of a medium, usually one containing Brønsted acids:17 Tendency of the medium to hydronate a specific reference base.18 Note 1: The acidity of a medium is quantitatively expressed by the appropriate acidity 19 function.20 Note 2: Media having an acidity greater than that of 100 % H2SO4 are often called 21 superacids.22 See [18].23 rev[3]2425 acidity function26 Measure of the thermodynamic hydron-donating or -accepting ability of a solvent 27 system, or a closely related thermodynamic property, such as the tendency of the lyate 28 ion of the solvent system to form Lewis adducts.29 Note 1: Acidity functions are not unique properties of the solvent system alone but 30 depend on the solute (or family of closely related solutes) with respect to which the 31 thermodynamic tendency is measured.32 Note 2: Commonly used acidity functions are extensions of pH to concentrated acidic 33 or basic solutions. Acidity functions are usually established over a range of 34 compositions of such a system by UV/Vis spectrophotometric or NMR measurements 35 of the degree of hydronation (or Lewis adduct formation) for the members of a series of 36 structurally similar indicator bases (or acids) of different strength: the best known of 37 these is the Hammett acidity function Ho (for primary aromatic amines as indicator 38 bases).39 For detailed information on other acidity functions, on excess acidity, on the 40 evaluation of acidity functions, and on the limitations of the concept, see [19,20,21,22].

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1 [3]23 activated complex4 See activated state.5 rev[3]67 activated state 8 In theories of unimolecular reactions an energized chemical species, often 9 characterized by the superscript ‡, where the excitation is specific and the molecule is

10 poised for reaction.11 Note 1: Often used as a synonym for activated complex or transition state, but not 12 restricted to transition-state theory.13 Note 2: This is distinct from an energized molecule, often characterized by the 14 superscript *, in which excitation energy is dispersed among internal degrees of 15 freedom.16 Note 3: This is not a complex according to the definition in this Glossary.17 See also transition state, transition structure.1819 activation energy 20 See energy of activation.21 See [12], section 2.12.22 [3]

23

24 activation strain model25 See distortion interaction model.2627 addition reaction28 Chemical reaction of two or more reacting molecular entities, resulting in a product 29 containing all atoms of all components, with formation of two chemical bonds and a net 30 reduction in bond multiplicity in at least one of the reactants.31 Note 1: The addition to an entity may occur at only one site (1,1-addition, insertion), 32 at two adjacent sites (1,2-addition) or at two non-adjacent sites (1,3- or 1,4-addition, 33 etc.). 34 Examples3536 R3C–H + H2C: → R3C–CH3 (1,1-addition of a carbene)37 Br2 + CH2=CH–CH=CH2 → BrCH2–CH(Br)–CH=CH2 (1,2-addition) + 38 BrCH2–CH=CH–CH2Br (1,4-addition)39

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1 Note 2: This is distinguished from adduct formation, which is less specific about 2 bonding changes.3 Note 3: The reverse process is called an elimination reaction.4 See also cheletropic reaction, cycloaddition, insertion.5 rev[3]67 additivity principle8 Hypothesis that each of several structural features of a molecular entity makes an 9 independent, transferable, and additive contribution to a property of the substance

10 concerned.11 Note 1: More specifically, it is the hypothesis that each of the several substituent 12 groups in a parent molecule makes a separate and additive contribution to the standard 13 Gibbs energy change or Gibbs energy of activation corresponding to a particular 14 chemical reaction.15 Note 2: The enthalpies of formation of series of compounds can be described by 16 additivity schemes [23].17 Note 3: Deviations from additivity may be remedied by including terms describing 18 interactions between atoms or groups.19 See transferability [24,25].20 rev[3]2122 adduct23 New chemical species AB, each molecular entity of which is formed by direct 24 combination of two (or more) separate molecular entities A and B in such a way that 25 there is no loss of atoms from A or B.26 Example: adduct formed by interaction of a Lewis acid with a Lewis base:27 (CH3)3N+––BF3

28 Note 1: Stoichiometries other than 1:1 are also possible, e.g., a bis-adduct (2:1). 29 Note 2: An intramolecular adduct can be formed when A and B are groups contained 30 within the same molecular entity.31 Note 3: If adduct formation is prevented by steric hindrance, frustrated Lewis pairs 32 may result. 33 See also addition reaction, frustrated Lewis pair, Lewis adduct, Meisenheimer 34 adduct, -adduct.35 rev[3]3637 agostic38 Feature of a structure in which a hydrogen atom is bonded to both a main-group atom 39 and a metal atom.40 Example, [(1–3-)-but-2-en-1-yl-2-C4,H4]( 5-cyclopentadienyl)cobalt(+1).

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123456 Note: The expression -hydrido-bridged is also used to describe the bonding 7 arrangement with a bridging hydrogen, but this usage is deprecated.8 See [26,27,28,29].9 rev[3]

1011 alcoholysis12 Reaction with an alcohol solvent.13 Examples:1415 C3H5(OCOR)3 (triglyceride) + 3 CH3OH → C3H5(OH)3 (glycerol) + 3 RCOOH16 (CH3)3CCl + CH3OH → (CH3)3COCH3 + H+ + Cl–1718 See solvolysis.19 rev[3]202122 allotropes23 Different structural modifications of an element, with different bonding arrangements of 24 the atoms.25 Examples for carbon include diamond, fullerenes, graphite, and graphene.26 See [29].2728 allylic substitution reaction29 Substitution reaction on an allylic system with leaving group (nucleofuge) at position 1 30 and double bond between positions 2 and 3. The incoming group may become attached 31 to atom 1, or else the incoming group may become attached at position 3, with 32 movement of the double bond from 2,3 to 1,2.33 Example:34 CH3CH=CHCH2Br + HOAc → CH3CH=CHCH2OAc or CH3CH(OAc)CH=CH2

3536 Note: This term can be extended to systems such as: 3738 CH3CH=CH–CH=CH–CH2Br → CH3CH(OAc)–CH=CH–CH=CH2 + 39 CH3CH=CH–CH(OAc)–CH=CH2 + CH3CH=CH–CH=CH–CH2OAc40

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1 and also to propargylic substitution, with a triple bond between positions 2 and 3 and 2 possible rearrangement to an allenic product.3 rev[3]45 alternant 6 Property of a conjugated system of electrons whose atoms can be divided into two 7 sets (marked as "starred" and "unstarred") so that no atom of either set is directly linked 8 to any other atom of the same set.9 Examples

1011121314151617 Note 1: According to several approximate theories (including HMO theory), the 18 MOs for an alternant hydrocarbon are paired, such that for an orbital of energy + x 19 there is another of energy x. The coefficients of paired molecular orbitals at each 20 atom are the same, but with opposite sign for the unstarred atoms, and the electron 21 density at each atom in a neutral alternant hydrocarbon is unity.22 See [30,31,32].23 rev[3]2425 ambident26 ambidentate, bidentate27 Characteristic of a chemical species whose molecular entities each possess two 28 alternative, distinguishable, and strongly interacting reactive centres, to either of which 29 a bond may be made.30 Note 1: Term most commonly applied to conjugated nucleophiles, for example an 31 enolate ion (which may react with electrophiles at either the -carbon atom or the 32 oxygen) or a 4-pyridone, and also to the vicinally ambident cyanide ion and to cyanate 33 ion, thiocyanate ion, sulfinate ions, nitrite ion, and unsymmetrical hydrazines.34

35 3637 Note 2: Ambident electrophiles are exemplified by carboxylic esters RC(O)OR', 38 which react with nucleophiles at either the carbonyl carbon or the alkoxy carbon, and

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1 by Michael acceptors, such as enones, that can react at either the carbonyl or the -2 carbon.3 Note 3: Molecular entities containing two non-interacting (or feebly interacting) 4 reactive centres, such as dianions of dicarboxylic acids, are not generally considered 5 to be ambident or bidentate and are better described as bifunctional.6 Note 4: The Latin root of the word implies two reactive centres, but the term has also 7 been applied to chemical species with more than two reactive centres, such as an acyl 8 thiourea, RCONHCSNHR', with nucleophilic O, S, and N. For such species the term 9 polydentate (or multidentate) is more appropriate.

10 See [33,34,35,36]. 11 [3]1213 ambiphilic14 Both nucleophilic and electrophilic.15 Example:16 CH2=O17 See also amphiphilic, but the distinction between ambi (Latin: both) and amphi 18 (Greek: both) and the application to hydrophilic and lipophilic or to nucleophilic and 19 electrophilic is arbitrary.2021 aminoxyl22 Compound having the structure

23 R2N–O• R2N•+–O–

24 Note: The synonymous term 'nitroxyl radical' erroneously suggests the presence of 25 a nitro group; its use is deprecated.2627 amphiphilic28 Both hydrophilic and lipophilic, owing to the presence in the molecule of a large organic 29 cation or anion and also a long hydrocarbon chain (or other combination of polar and 30 nonpolar groups, as in nonionic surfactants)31 Examples:3233 CH3(CH2)nCO2

–M+, CH3(CH2)nSO3–M+, CH3(CH2)nN(CH3)3

+X– (n > 7).3435 Note: The presence of distinct polar (hydrophilic) and nonpolar (hydrophobic) regions 36 promotes the formation of micelles in dilute aqueous solution.37 See also ambiphilic.38 [3]3940 amphiprotic (solvent)

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1 Feature of a self-ionizing solvent possessing characteristics of both Brønsted acid and 2 base.3 Examples: H2O, CH3OH.4 [3]56 amphoteric7 Property of a chemical species that can behave as either an acid or as a base.8 Examples: H2O, HCO3

– (hydrogen carbonate)9 Note: This property depends upon the medium in which the species is investigated.

10 For example HNO3 is an acid in water but becomes a base in H2SO4.11 rev[3]1213 anchimeric assistance14 neighbouring group participation15 [3]1617 anionotropy 18 Rearrangement or tautomerization in which the migrating group moves with its electron 19 pair.20 See [37].21 [3]2223 annelation24 Alternative, but less desirable term for annulation.25 Note: The term is widely used in German and French languages.26 [3]2728 annulation29 Transformation involving fusion of a new ring to a molecule via two new bonds.30 Note: Some authors use the term annelation for the fusion of an additional ring to an 31 already existing one, and annulation for the formation of a ring from an acyclic 32 precursor.33 See [38,39].34 See also cyclization.35 [3]3637 annulene38 Conjugated monocyclic hydrocarbon of the general formula CnHn (n even) with the 39 maximum number of noncumulative double bonds and without side chains.

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1 Note: In systematic nomenclature an annulene may be named [n]annulene, where n 2 is the number of carbon atoms, e.g., [8]annulene for cycloocta-1,3,5,7-tetraene.3 See [40].4 See aromatic, Hückel (4n + 2) rule.5 [3] 67 anomeric effect8 Tendency of an electronegative substituent alpha to a heteroatom in a six-membered 9 ring to prefer the axial position, as in the anomers of glucopyranose.

10 Note 1: The effect can be generalized to the conformational preference of an 11 electronegative substituent X to be antiperiplanar to a lone pair of atom Y in a system 12 R–Y–C–X with geminal substituents RY and X. 13 Note 2: The effect can be attributed, at least in part, to n-* delocalization of the lone 14 pair on Y into the C–X * orbital.15 See [11,41,42,43,44]. 1617 anomers18 The two stereoisomers (epimers) of a cyclic sugar or glycoside that differ only in the 19 configuration at C1 of aldoses or C2 of ketoses (the anomeric or acetal/ketal carbon).20 Example:2122232425 See [11].26 [3]2728 antarafacial, suprafacial29 Two spatially different ways whereby bonding changes can occur when a part of a 30 molecule undergoes two changes in bonding (bond-making or bond-breaking), either 31 to a common centre or to two related centres external to itself. These are designated 32 as antarafacial if opposite faces of the molecule are involved, and suprafacial if both 33 changes occur at the same face. The concept of face is clear from the diagrams in the 34 cases of planar (or approximately planar) frameworks with interacting orbitals.353637383940

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123 For examples of the use of these terms see cycloaddition, sigmatropic rearrangement.4 See also anti, , .5 rev[3]67 anti8 Stereochemical relationship of two substituents that are on opposite sides of a 9 reference plane, in contrast to syn, which means "on the same side".

10 Note 1: Two substituents attached to atoms joined by a single bond are anti if the 11 torsion angle (dihedral angle) between the bonds to the substituents is greater than 90º, 12 in contrast to syn if it is less than 90º. 13 Note 2: A further distinction is made between antiperiplanar, synperiplanar, anticlinal 14 and synclinal.15 See [11,45,46,47].16 Note 3: When the terms are used in the context of chemical reactions or 17 transformations, they designate the relative orientation of substituents in the substrate 18 or product:19 (1) Addition to a carbon-carbon double bond:2021222324252627282930 (2) Alkene-forming elimination: 31

323334 (3) Aldol reaction (where syn and anti designate the relative orientation of CH3 35 and OH in the product)36

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12 Note 4: In examples (1) and (2) anti processes are always antarafacial, and syn 3 processes are suprafacial.4 Note 5: In the older literature the terms anti and syn were used to designate 5 stereoisomers of oximes and related compounds. That usage was superseded by the 6 terms trans and cis or E and Z.7 See [11].8 rev[3]9

10 anti-Hammond effect11 If a structure lying off the minimum-energy reaction path (MERP) is stabilized, the 12 position of the transition state moves toward that structure. 13 See Hammond Postulate, More O'Ferrall - Jencks diagram, perpendicular effect.14 rev[3]1516 aprotic (solvent)17 non-HBD solvent (non-Hydrogen-Bond Donating solvent)18 Solvent that is not capable of acting as a hydrogen-bond donor.19 Note: Although this definition applies to both polar and nonpolar solvents, the 20 distinction between HBD and non-HBD (or between protic and aprotic) is relevant only 21 for polar solvents.22 See HBD solvent.23 rev[3]242526 aquation27 Incorporation of one or more integral molecules of water into a species, with or without 28 displacement of one or more other atoms or groups.29 Example: The incorporation of water into the inner ligand sphere of an inorganic 30 complex.31 See [48].32 See also hydration.33 [3]

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12 aromatic (adj.), aromaticity (n.)3 (1) Having a chemistry typified by benzene (traditionally).4 (2) Feature of a cyclically conjugated molecular entity whose electronic energy is 5 significantly lower or whose stability is significantly greater (owing to delocalization) 6 than that of a hypothetical localized structure (e.g., Kekulé structure). 7 Note 1: If the molecular entity is of higher energy or less stable than a hypothetical 8 localized structure, the entity is said to be antiaromatic.9 Note 2: A geometric parameter indicating bond-length equalization has been used

10 as a measure of aromatic character, as expressed in the harmonic oscillator model of 11 aromaticity [49]. 12 Note 3: The magnitude of the magnetically induced ring current, as observed 13 experimentally by NMR spectroscopy or by the calculated nucleus-independent 14 chemical shift (NICS) value, is another measure of aromaticity [50]. 15 Note 4: The terms aromatic and antiaromatic have been extended to describe the 16 stabilization or destabilization of transition states of pericyclic reactions. The 17 hypothetical reference structure is here less clearly defined, and use of the term is 18 based on application of the Hückel (4n + 2) rule and on consideration of the topology of 19 orbital overlap in the transition state, whereby a cycle with (4n+2) electrons and a 20 Möbius cycle with 4n electrons are aromatic. Reactions of molecules in the ground state 21 involving antiaromatic transition states proceed much less easily than those involving 22 aromatic transition states. 23 See [51,52,53]. See 19 articles in [54].24 See also Hückel (4n + 2) rule, Möbius aromaticity.25 [3]2627 Arrhenius equation28 Empirical expression for the temperature dependence of a reaction rate constant k as29 k(T) = A exp(–EA/RT),30 with A the pre-exponential factor (Arrhenius A factor) and EA the Arrhenius energy of 31 activation, both considered to be temperature-independent.32 rev[3]3334 aryne35 Hydrocarbon derived from an arene by formal removal of two vicinal hydrogen atoms.36 Example: 1,2-didehydrobenzene (benzyne)37

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123 Note 1: 1,4-Didehydrobenzene ("p-benzyne diradical", structure above right). 4 Despite common usage this is not an aryne because there is no triple bond, and the 5 usage is deprecated.6 Note 2: Arynes are usually transient species.7 Note 3: The analogous heterocyclic compounds are called heteroarynes or 8 hetarynes.9 See [40,55].

10 rev[3]1112 association13 Assembling of separate molecular entities into any aggregate, especially of oppositely 14 charged free ions into ion pairs or larger and not necessarily well-defined clusters of 15 ions held together by electrostatic attraction.16 Note: The term signifies the reverse of dissociation, but is not commonly used for the 17 formation of definite adducts by colligation or coordination.18 [3]1920 asymmetric induction21 Preferential formation in a chemical reaction of one enantiomer or diastereoisomer over 22 the other as a result of the influence of a chiral centre (stereogenic centre, chiral feature) 23 in the substrate, reagent, catalyst, or environment.24 Note: The term also refers to the formation of a new chiral centre or chiral feature 25 preferentially in one configuration under such influence.26 See [11].27 rev[3]2829 atomic charge30 Net charge due to the nucleus and the average electronic distribution in a given region 31 of space. This region is considered to correspond to an atom in a molecular entity.32 Note 1: The boundary limits of an atom in a polyatomic molecular entity cannot be 33 defined, as they are not a quantum-mechanical observable. Therefore, different 34 conceptual schemes of dividing a molecule into individual atoms will result in different 35 atomic charges.36 Note 2: The atomic charge on an atom should not be confused with its formal charge. 37 For example, the N in NH4

+ is calculated to carry a net negative charge even though its

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1 formal charge is +1, and each H is calculated to carry a net positive charge even though 2 its formal charge is 0.3 See [8].45 atomic orbital6 Wavefunction that depends explicitly on the spatial coordinates of only one electron 7 around a single nucleus.8 See also molecular orbital.9 See [8].

10 [3]1112 atropisomers13 Stereoisomers that are enantiomeric owing to hindered rotation about a single bond.14 Example (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl, BINAP):1516171819202122 See [11].

2324 attachment25 Transformation by which one molecular entity (the substrate) is converted into another 26 by the formation of one (and only one) two-centre bond between the substrate and 27 another molecular entity and which involves no other changes in connectivity in the 28 substrate.29 Example: formation of an acyl cation by attachment of carbon monoxide to a 30 carbenium ion (R+):31 R+ + CO → (RCO)+

32 See also colligation.33 rev[3]3435 autocatalytic reaction36 Chemical reaction in which a product (or a reaction intermediate) also functions as 37 catalyst.38 Note: In such a reaction the observed rate of reaction is often found to increase with 39 time from its initial value.

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1 Example: acid-catalyzed bromination of acetophenone, PhCOCH3, because the 2 reaction generates HBr, which functions as a catalyst. 3 rev[3]45 automerization6 degenerate rearrangement7 [3]89 autoprotolysis

10 Proton transfer reaction (hydron transfer reaction) between two identical amphoteric 11 molecules (usually of a solvent), one acting as a Brønsted acid and the other as a 12 Brønsted base.13 Example:14 2 H2O → H3O+ + HO–

15 [3]1617 autoprotolysis constant18 Product of the activities of the species produced as the result of autoprotolysis.19 Example: The autoprotolysis constant for water, Kw (or KAP), is equal to the product 20 of the relative activities of the hydronium and hydroxide ions at equilibrium in pure water.2122 Kw = a(H3O+)a(HO–) = 1.0 10–14 at 25 °C and 1 standard atmosphere.2324 Note: Since the relative activity a(H2O) of water at equilibrium is imperceptibly 25 different from unity (with mole fraction as the activity scale and pure un-ionized water 26 as the standard state), the denominator in the expression for the thermodynamic 27 equilibrium constant Kw° for autoprotolysis has a value very close to 1.28

29 Kw° = a(H3O + )a(HO–)

a(H2O)2

3031 Furthermore, owing to the low equilibrium extent of dissociation, and if infinite dilution 32 is selected as the standard state, the activity coefficients of the hydronium and 33 hydroxide ions in pure water are very close to unity. This leads to the relative activities 34 of H3O+ and HO being virtually identical with the numerical values of their molar 35 concentrations, if the molar scale of activity is used and an activity of 1 mol dm3 is 36 chosen as the standard state. Thus 3738 Kw [H3O+][HO]39

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1 where [H3O+] and [HO] are the numerical values of the molar concentrations. The 2 autoprotolysis constant has the unit (and dimension) 1 because each relative activity 3 has dimension 1, being a quotient of an absolute activity (including units) divided by a 4 common unit standard state activity (including units).5 See [56,57].6 rev[3]78 (alpha)9 (1) Designation applied to the carbon to which a functional group is attached.

10 (2) In carbohydrate nomenclature a stereochemical designation of the configuration at 11 the anomeric carbon.12 (3) Parameter in a Brønsted relation expressing the sensitivity of the rate of protonation 13 to acidity. 14 (4) Parameter in Leffler's relation expressing the sensitivity of changes in Gibbs 15 activation energy to changes in overall Gibbs energy for an elementary reaction.16 See [11].17 rev[3]1819 -effect20 Positive deviation of an nucleophile (one bearing an unshared pair of electrons on an 21 atom adjacent to the nucleophilic site) from a Brønsted-type plot of lg {knuc} vs. pKa. The 22 argument in the lg function should be of dimension 1. Thus, reduced rate coefficients 23 should be used. Here {knuc} = knuc/[knuc] is the reduced knuc.24 Note 1: More generally, it is the influence on the reactivity at the site adjacent to the 25 atom bearing a lone pair of electrons.26 Note 2: The term has been extended to include the effect of any substituent on an 27 adjacent reactive centre, e.g., the -silicon effect.28 See [58,59,60]. 29 See also Brønsted relation.30 rev[3]3132 -elimination33 1,1-elimination34 Transformation of the general type3536 RR'ZXY → RR'Z + XY (or X + Y, or X+ + Y–)3738 where the central atom Z is commonly carbon.39 See also elimination.40 rev[3]

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12 Baldwin's rules3 Set of empirical rules for closures of 3- to 7-membered rings.4 Note: The favoured pathways are those in which the length and nature of the linking 5 chain enable the terminal atoms to achieve the proper geometry and orbital overlaps 6 for reaction.7 Example: Hex-5-en-1-yl radical undergoes 5-exo-trig cyclization to cyclopentylmethyl 8 radical rather than 6-endo-trig cyclization to cyclohexyl radical.9

10 CH2

CH2 not

1112 See [61,62]. 13 rev[3]1415 base16 Chemical species or molecular entity having an available pair of electrons capable of 17 forming a bond with a hydron (proton) (see Brønsted base) or with the vacant orbital of 18 some other species (see Lewis base).19 See also hard base, superbase.20 [3]2122 basicity 23 (1) Tendency of a Brønsted base to act as hydron (proton) acceptor.24 Note 1: The basicity of a chemical species is normally expressed by the acidity or 25 acid-dissociation constant of its conjugate acid (see conjugate acid-base pair). 26 Note 2: To avoid ambiguity, the term pKaH should be used when expressing basicity 27 by the acid-dissociation constant of its conjugate acid. Thus the pKaH of NH3 is 9.2, 28 while its pKa, expressing its acidity, is 38. 29 (2) Tendency of a Lewis base to act as a Lewis acid acceptor.30 Note 3: For Lewis bases, basicity is expressed by the association constants of Lewis 31 adducts and -adducts, or by the enthalpy of an acid/base reaction.32 Note 4: Spectroscopic shifts induced by acid/base adduct formation can also be used 33 as a measure of the strength of interaction.34 See [63].35 rev[3]3637 bathochromic shift (effect)38 Shift of a spectral band to lower frequencies (longer wavelengths).

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1 Note: This is informally referred to as a red shift and is opposite to a hypsochromic 2 shift ("blue shift"), but these historical terms are discouraged because they apply only 3 to visible transitions.4 See [9].5 [3]67 Bell-Evans-Polanyi principle8 Linear relation between energy of activation (EA) and enthalpy of reaction (rH), 9 sometimes observed within a series of closely related reactions.

1011 EA = a + b rH 1213 See [64,65,66,67].14 rev[3]1516 benzyne17 1,2-Didehydrobenzene (a C6H4 aryne derived from benzene) and its derivatives formed 18 by substitution.19 Note: The terms m- and p-benzyne are occasionally used for 1,3- and 1,4-20 didehydrobenzene, respectively, but these are incorrect because there is no triple bond.21 See [40,55].22 [3]2324 bidentate25 Feature of a ligand with two potential binding sites.2627 bifunctional catalysis28 Catalysis (usually of hydron transfer) by a chemical species involving a mechanism in 29 which two functional groups are implicated in the rate-limiting step, so that the 30 corresponding catalytic coefficient is larger than that expected for catalysis by a 31 chemical species containing only one of these functional groups.32 Example: Hydrogen carbonate is a particularly effective catalyst for the hydrolysis of 33 4-hydroxybutyranilide (N-phenyl-4-hydroxybutanamide), because it catalyzes the 34 breakdown of the tetrahedral intermediate to expel aniline: 35

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123 Note: The term should not be used to describe a concerted process involving the 4 action of two different catalysts.5 See [68,69,70,71,72].6 rev[3]78 bifurcation9 Feature on a potential-energy surface whereby a minimum-energy reaction path

10 (MERP) emanating from a saddle point (corresponding to a transition structure) splits 11 in two and leads to alternative products without intervening minima or secondary 12 barriers to overcome. A bifurcation arises when the curvature of the surface in a 13 direction perpendicular to the MERP becomes zero and then negative; it implies the 14 existence of a lower-energy transition structure with a transition vector orthogonal to 15 the original MERP.16 See [73,74].1718 bimolecular19 See molecularity.20 [3]2122 binding site23 Specific region (or atom) in a molecular entity that is capable of entering into a 24 stabilizing interaction with another atomic or molecular entity.25 Example: an active site in an enzyme that interacts with its substrate.26 Note 1: Typical modes of interaction are by covalent bonding, hydrogen bonding, 27 coordination, and ion-pair formation, as well as by dipole-dipole interactions, dispersion 28 forces, hydrophobic interactions, and desolvation.29 Note 2: Two binding sites in different molecular entities are said to be complementary 30 if their interaction is stabilizing.31 [3]32

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1 biradical2 diradical3 See [9].4 rev[3]56 blue shift7 Informal expression for hypsochromic shift, but this historical term is discouraged 8 because it applies only to visible transitions.9 rev[3]

1011 Bodenstein approximation12 See steady state.13 [3]1415 bond16 Balance of attractive and repulsive forces between two atoms or groups of atoms, 17 resulting in sufficient net stabilization to lead to the formation of an aggregate 18 conveniently considered as an independent molecular entity.19 Note: The term usually refers to the covalent bond.20 See [75]. 21 See also agostic, coordination, hydrogen bond, multi-centre bond.22 rev[3]2324 bond dissociation25 See heterolysis, homolysis.26 Note: In ordinary usage the term refers to homolysis. If not, it should be specified as 27 heterolytic.28 [3]2930 bond-dissociation energy31 De

32 (SI unit: kJ mol–1)33 Energy required to break a given bond of some specific molecular entity by homolysis 34 from its potential-energy minimum.35 Note: This is the quantity that appears in the Morse potential.36 See also bond-dissociation enthalpy.37 rev[3]3839 bond-dissociation enthalpy40 DH0, dissH0

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1 (SI unit: kJ mol–1)2 Standard molar enthalpy required to break a given bond of some specific molecular 3 entity by homolysis.4 Example: For CH4 → CH3· + H· the bond-dissociation enthalpy is symbolized as 5 DH0(CH3–H).6 Note: Although DH0 is commonly used, dissH0 is more consistent with the notation 7 for other thermodynamic quantities.8 See also bond-dissociation energy, bond energy, heterolytic bond-dissociation 9 enthalpy.

1011 bond energy12 D0

13 (SI unit: kJ mol–1)14 Enthalpy of bond dissociation at 0 K.15 See bond-dissociation enthalpy.16 rev[3]1718 bond enthalpy (mean bond enthalpy, mean bond energy)19 Average value of the gas-phase bond-dissociation enthalpies (usually at a temperature 20 of 298 K) for all bonds of the same type within the same chemical species.21 Example: For methane, the mean bond enthalpy is 415.9 kJ mol–1, one-fourth the 22 enthalpy of reaction for23 CH4(g) → C(g) + 4 H(g)24 Note: More commonly, tabulated mean bond energies (which are really enthalpies) 25 are values of bond enthalpies averaged over a number of selected chemical species 26 containing that type of bond, such as 414 kJ mol–1 for C–H bonds in a group of R3CH 27 (R = H, alkyl).28 See [76]. 2930 bond order31 Theoretical index of the degree of bonding between two atoms, relative to that of a 32 normal single bond, i.e., the bond provided by one localized electron pair.33 Example: In ethene the C–C bond order is 2, and the C–H bond order is 1.34 Note 1: In valence-bond theory it is a weighted average of the bond orders between 35 the respective atoms in the various resonance forms. In molecular-orbital theory it is 36 calculated from the weights of the atomic orbitals in each of the occupied molecular 37 orbitals. For example, in valence-bond theory the bond order between adjacent carbon 38 atoms in benzene is 1.5; in Hückel molecular orbital theory it is 1.67.39 Note 2: Bond order is often derived from the electron distribution.

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1 Note 3: The Pauling bond order n (as often used in the bond-energy-bond-order 2 model) is a simple function of change in bond length d, where the value of the coefficient 3 c is often 0.3 Å (for n > 1) or 0.6 Å (for n < 1).45 n = exp[(d1 – dn)/c]6 rev[3]78 bond-energy-bond-order model (BEBO):9 Empirical procedure for estimating activation energy, involving relationships among

10 bond length, bond-dissociation energy, and bond order.11 See [13,77]. 1213 bond-stretch isomers14 Two (or more) molecules with the same spin multiplicity but with different lengths for 15 one or more bonds.16 Note: This feature arises because the potential-energy surface, which describes how 17 the energy of the molecule depends on geometry, shows two (or more) minima that are 18 not merely symmetry-related. 19 See [78,79,80,81].2021 borderline mechanism22 Mechanism intermediate between two extremes, for example a nucleophilic substitution 23 intermediate between SN1 and SN2, or intermediate between electron transfer and SN2.24 [3]2526 Born-Oppenheimer approximation27 Representation of the complete wavefunction as a product of electronic and nuclear 28 parts, (r,R) = el(r,R) nuc(R), so that the two wavefunctions can be determined 29 separately by solving two different Schrödinger equations.30 See [8].3132 Bredt's rule33 Prohibition of placing a double bond with one terminus at the bridgehead atom of a 34 polycyclic system unless the rings are large enough to accommodate the double bond 35 without excessive strain.36 Example: Bicyclo[2.2.1]hept-1-ene (A), which is capable of existence only as a 37 transient species, although its higher homologues, bicyclo[3.3.1]non-1-ene (B) and 38 bicyclo[4.2.1]non-1(8)-ene (C), with double bond at the bridgeheads, have been 39 isolated.40

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1234 A B C 56 See [82,83,84,85].7 For limitations see [86].8 Note: For an alternative formulation, based on the instability of a trans double bond 9 in a small ring (fewer than 8 atoms), see [87].

10 rev[3]1112 bridged carbocation13 Carbocation (real or hypothetical) in which there are two (or more) carbon atoms that 14 could in alternative Lewis formulas be designated as carbenium centres but which is 15 instead represented by a structure in which a group (a hydrogen atom or a hydrocarbon 16 residue, possibly with substituents in non-involved positions) bridges these potential 17 carbenium centres.18 Note: Electron-sufficient bridged carbocations are distinguished from electron-19 deficient bridged carbocations. Examples of the former, where the bridging uses 20 electrons, are phenyl-bridged ions (for which the trivial name phenonium ion has been 21 used), such as A. These ions are straightforwardly classified as carbenium ions. The 22 latter type of ion necessarily involves three-centre bonding because the bridging uses 23 σ electrons. The hydrogen-bridged carbocation B contains a two-coordinate hydrogen 24 atom, whereas structures C, D, and E (the 2-norbornyl cation) contain five-coordinate 25 carbon atoms. 26

272829 See [88].30 See also carbonium ion, multi-centre bond, neighbouring group participation.31 For the definitive X-ray structure of the norbornyl cation see [89].32 rev[3]33343536 bridgehead (atom)

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1 Atom that is part of two or more rings in a polycyclic molecule and that is separated 2 from another bridgehead atom by bridges all of which contain at least one other atom.3 Example: C1 and C4 in bicyclo[2.2.1]heptane, but not C4a and C8a in 4 decahydronaphthalene.56 bridging ligand7 Ligand attached to two or more, usually metallic, central atoms.8 See [29].9 [3]

1011 Brønsted acid (Brönsted acid)12 Molecular entity capable of donating a hydron (proton) to a base (i.e., a hydron donor), 13 or the corresponding chemical species.14 Examples: H2O, H3O+, CH3COOH, H2SO4, HSO4

–, HCl, CH3OH, NH3.15 See also conjugate acid-base pair.16 [3]1718 Brønsted base (Brönsted base)19 Molecular entity capable of accepting a hydron (proton) from a Brønsted acid (i.e., a 20 hydron acceptor), or the corresponding chemical species.21 Examples: HO–, H2O, CH3CO2

–, HSO4–, SO4

2–, Cl–, CH3O–, NH2–.

22 See also conjugate acid-base pair.23 [3]2425 Brønsted relation (Brönsted relation)26 Either of the equations2728 lg({kHA}/p) = C + lg(q{KHA}/p)2930 lg({kA}/q) = C – lg(q{KHA}/p)3132 where, , and C are constants for a given reaction series ( and are called Brønsted 33 exponents or Brønsted parameters). The arguments in the lg functions should be of 34 dimension 1. Thus, reduced rate coefficients should be used: {kA} = kA/[kA] and {kHA} = 35 kHA/[kHA] , which are the reduced catalytic coefficients of reactions whose rates depend 36 on the concentrations of acid HA or of its conjugate base A–, {KHA} = KHA/[KHA] is the 37 reduced acid dissociation constant of HA, p is the number of equivalent acidic protons 38 in HA, and q is the number of equivalent basic sites in A–. The chosen values of p and 39 q should always be specified. (The charge designations of HA and A– are only 40 illustrative.).

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1 Note1: The equations are often written without reduced variables, whereupon the 2 slope or , obtained from a graph or least-squares analysis, is correct because it is 3 the derivative of a logarithmic quantity, of dimension 1.4 Note 2: The Brønsted relation is often termed the Brønsted catalysis law. Although 5 justifiable on historical grounds, use of this name is not recommended, since Brønsted 6 relations are known to apply to many uncatalysed and pseudo-catalysed reactions 7 (such as simple proton [hydron] transfer reactions).8 Note 3: The term pseudo-Brønsted relation is sometimes used for reactions that 9 involve nucleophilic catalysis instead of acid-base catalysis. Various types of Brønsted

10 parameters have been proposed such as nuc or lg for nucleophile or leaving group, 11 respectively. 12 See also linear free-energy relation (linear Gibbs energy relation).13 rev[3]1415 Bunnett-Olsen equations16 Relations between lg([SH+]/[S]) + Ho and Ho + lg{[H+]} for base S in aqueous acid 17 solutions, where Ho is Hammett's acidity function and Ho + lg{[H+]} represents the 18 activity function lg({S} {H+} / {SH+}) for the nitroaniline reference bases to build Ho. and 19 where is an empirical parameter that is determined by the slope of the linear 20 correlation of lg([SH+]/[S]) – lg{[H+]} vs. Ho + lg{[H+]}.2122 lg([SH+]/[S]) – lg{[H+]} = ( – 1)(Ho + lg{[H+]}) + pKSH+

2324 lg([SH+]/[S]) + Ho = (Ho + lg{[H+]}) + pKSH+

2526 Arguments in the lg functions should be of dimension 1. Thus, concentrations should 27 be divided by the respective unit (unless they are eliminated as in the ratio of two 28 concentrations), i.e., the reduced quantity should be used, indicated by curly brackets.29 Note 1: These equations avoid using (or defining) an acidity function for each family 30 of bases, including those for which such a definition is not possible. In many cases, 31 (or – 1) values for base families defining an acidity function are very similar. Broadly, 32 the value of is related to the degree of solvation of SH+.33 Note 2: These equations are obsolete, and the Cox-Yates equation, with the 34 equivalent parameter m* (= 1 – ), is now preferred.35 See [21,90]. 36 See also Cox-Yates equation.37 rev[3]3839 , nuc, lg

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1 Parameter in a Brønsted relation expressing the sensitivity of the rate of deprotonation 2 to basicity.3 Note: nuc and lg are used to correlate nucleophilic reactivity and leaving-group 4 ability, respectively.56 -elimination7 1,2-elimination8 Transformation of the general type9

10 XABY → A=B + XY (or X + Y, or X+ + Y–)1112 where the central atoms A and B are commonly, but not necessarily, carbon.13 See also elimination.1415 cage16 Aggregate of molecules, generally solvent molecules in the condensed phase, that 17 surrounds fragments formed by thermal or photochemical dissociation.18 Note: Because the cage hinders the separation of the fragments by diffusion, they 19 may preferentially react with one another ("cage effect") although not necessarily to re-20 form the precursor species.21 Example:

2223 See also geminate recombination.24 [3]2526 cage compound27 Polycyclic compound capable of encapsulating another compound28 Example (adamantane, where the central cavity is large enough to encapsulate He, 29 Ne, or Na+):303132333435 Note: A compound whose cage is occupied is called an inclusion complex.36 See [91,92].37 rev[3]383940 canonical form

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1 resonance form2 rev[3]34 captodative effect5 Combined action of an electron-withdrawing ("captor") substituent and an electron-6 releasing ("dative") substituent, both attached to a radical centre, on the stability of a 7 carbon-centred radical.8 Example:9

1011121314 See [93,94,95].15 rev[3]1617 carbanion18 Generic name for an anion containing an even number of electrons and having an 19 unshared pair of electrons on a carbon atom which satisfies the octet rule and bears a 20 negative formal charge in its Lewis structure or in at least one of its resonance forms.21 Examples:

22H C C CH3 CH2

O

CH3 CH2

O

2324 See also radical ion.25 See [40,96].26 rev[3]2728 carbene29 Generic name for the species H2C: ("methylidene") and substitution derivatives thereof, 30 containing an electrically neutral bivalent carbon atom with two nonbonding electrons.31 Note 1: The nonbonding electrons may have antiparallel spins (singlet state) or 32 parallel spins (triplet state).33 Note 2: Use of the alternative name "methylene" as a generic term is not 34 recommended.35 See [40].36 See also diradical.37 [3]38

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1 carbenium ion2 Generic name for a carbocation whose electronic structure can be adequately 3 described by two-electron-two-centre bonds.4 Note 1: The name implies a hydronated carbene or a substitution derivative thereof.5 Note 2: The term is a replacement for the previously used term carbonium ion, which 6 now specifies a carbocation with penta- or higher-coordinate carbons. The names 7 provide a useful distinction between tricoordinate and pentacoordinate carbons.8 Note 3: To avoid ambiguity, the name should be avoided as the root for the 9 nomenclature of carbocations. For example, the term "ethylcarbenium ion" might refer

10 to either CH3CH2+ (ethyl cation, ethylium) or CH3CH2CH2

+ (propyl cation, propylium).11 See [40,97].12 rev[3]1314 carbenoid15 Carbene-like chemical species but with properties and reactivity differing from the free 16 carbene, arising from additional substituents bonded to the carbene carbon.17 Example: R1R2C(Cl)M (M = metal)18 rev[3]1920 carbocation21 Positive ion containing an even number of electrons and with a significant portion of the 22 excess positive charge located on one or more carbon atoms.23 Note 1: This is a general term embracing carbenium ions, all types of carbonium 24 ions, vinyl cations, etc.25 Note 2: Carbocations may be named by adding the word "cation" to the name of the 26 corresponding radical [97,98].27 Note 3: Such names do not imply structure (e.g., whether three-coordinated or five-28 coordinated carbon atoms are present) [98].29 See also bridged carbocation, radical ion.30 See [40].31 [3]3233 carbonium ion34 (1) Carbocation that contains at least one carbon atom with a coordination number of 35 five or greater.36 (2) Carbocation whose structure cannot adequately be described by only two-electron 37 two-centre bonds.38 Example: methanium (CH5

+).39 Note 1: In most of the earlier literature this term was used for all types of 40 carbocations, including those that are now defined as a (tricoordinate) carbenium ion.

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1 Note 2: To avoid ambiguity, the term should be avoided as the root for the 2 nomenclature of carbocations. For example, the name "ethylcarbonium ion" might refer 3 to either CH3CH2

+ (ethyl cation) or CH3CH2CH2+ (propyl cation).

4 See [98,99].5 rev[3]67 carbyne8 methylidyne 9 Generic name for the species HC·: and substitution derivatives thereof, such as

10 EtOCO–C·: (2-ethoxy-2-oxoethylidyne), containing an electrically neutral univalent 11 carbon atom with three non-bonding electrons.12 Note: Use of the alternative name "methylidyne" as a generic term is not 13 recommended.14 [3]1516 Catalán solvent parameters17 Quantitative measure of solvent polarity, based on the solvent’s hydrogen-bond-donor 18 ability, hydrogen-bond-acceptor ability, polarizability, and dipolarity.19 See [100,101].20 See also solvent parameter.2122 catalyst23 Substance that increases the rate of a chemical reaction (owing to a change of 24 mechanism to one having a lower Gibbs energy of activation) without changing the 25 overall standard Gibbs energy change (or position of equilibrium).26 Note 1: The catalyst is both a reactant and product of the reaction, so that there is 27 no net change in the amount of that substance.28 Note 2: At the molecular level, the catalyst is used and regenerated during each set 29 of microscopic chemical events leading from a molecular entity of reactant to a 30 molecular entity of product.31 Note 3: The requirement that there be no net change in the amount of catalyst is 32 sometimes relaxed, as in the base catalysis of the bromination of ketones, where base 33 is consumed, but this is properly called pseudo-catalysis.34 Note 4: Catalysis can be classified as homogeneous, in which only one phase is 35 involved, and heterogeneous, in which the reaction occurs at or near an interface 36 between phases.37 Note 5: Catalysis brought about by one of the products of a reaction is called 38 autocatalysis.39 Note 6: The terms catalyst and catalysis should not be used when the added 40 substance reduces the rate of reaction (see inhibition).

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1 Note 7: The above definition is adequate for isothermal-isobaric reactions, but under 2 other experimental conditions the state function that is lowered by the catalyst is not the 3 Gibbs activation energy but the quantity corresponding to those conditions (e.g., the 4 Helmholtz energy under isothermal-isochoric conditions).5 See [13].6 See also autocatalytic reaction, bifunctional catalysis, catalytic coefficient, electron-7 transfer catalysis, general acid catalysis, general base catalysis, intramolecular 8 catalysis, micellar catalysis, Michaelis-Menten kinetics, phase-transfer catalysis, 9 pseudo-catalysis, rate of reaction, specific catalysis.

10 rev[3]1112 catalytic antibody (abzyme)13 monoclonal antibody with enzymatic activity14 Note 1: A catalytic antibody acts by binding its antigen and catalyzing a chemical 15 reaction that converts the antigen into desired products. Despite the existence of natural 16 catalytic antibodies, most of them were specifically designed to catalyze desired 17 chemical reactions. 18 Note 2: Catalytic antibodies are produced through immunization against a transition-19 state analogue for the reaction of interest. The resulting antibodies bind strongly and 20 specifically the transition-state analogue, so that they become catalysts for the desired 21 reaction.22 Note 3: The concept of catalytic antibodies and the strategy for obtaining them were 23 advanced by W. P. Jencks [102]. The first catalytic antibodies were finally produced in 24 1986 [103,104].2526 catalytic coefficient27 If the rate of reaction v is expressible in the form2829 v = (k0 + ki[Ci]ni) [A]a [B]b...3031 where A, B, ... are reactants and Ci represents one of a set of catalysts, then the 32 proportionality factor ki is the catalytic coefficient of the particular catalyst Ci.33 Note: Normally the partial order of reaction (ni) with respect to a catalyst will be unity, 34 so that ki is an (a + b+...+1)th-order rate coefficient.35 [3]36373839 catalytic cycle

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1 Sequence of reaction steps in the form of a loop. One step is binding of a reactant to 2 the active catalyst (sometimes formed from a precatalyst), and another step is the 3 release of product and regeneration of catalyst.

4 Example: A + B → C, catalyzed by X formed from precatalyst X'.

56 cation/ interaction 7 Noncovalent attractive force between a positive ion (metal cation, protonated Brønsted 8 base, etc.) and a electron system.9 See [105].

1011 chain reaction12 Reaction in which one or more reactive reactive intermediates (frequently radicals) are 13 continuously regenerated through a repetitive cycle of elementary steps ("propagation 14 steps").15 Example: Chlorination of methane by a radical mechanism, where Cl· is continually 16 regenerated in the chain-propagation steps:17 Cl· + CH4 → HCl + H3C·18 H3C· + Cl2 → CH3Cl + Cl·19 Note: In chain polymerization reactions, reactive intermediates of the same types, 20 generated in successive steps or cycles of steps, differ in molecular mass, as in21 RCH2C·HPh + H2C=CHPh → RCH2CHPhCH2C·HPh22 See [106].23 See also chain transfer, initiation, termination.24 [3]2526 chain transfer27 Chemical reaction during a chain polymerization in which the active centre is transferred 28 from the growing macromolecule to another molecule or to another site on the same 29 molecule, often by abstraction of an atom by the radical end of the growing 30 macromolecule.

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1 Note 1: The growth of the polymer chain is thereby terminated but a new radical, 2 capable of chain propagation and polymerization, is simultaneously created. For the 3 example of alkene polymerization cited for a chain reaction, the reaction

4 RCH2C·HPh + CCl4 → RCH2CHClPh + Cl3C·5 represents a chain transfer, the radical Cl3C· inducing further polymerization:6 Note 2: Chain transfer also occurs in other chain reactions, such as cationic or 7 anionic polymerization, in which case the abstraction is by the reactive cationic or 8 anionic end of the growing chain.9 See [106].

10 See also telomerization.11 [3]1213 charge density14 See electron density.15 See [12].16 [3]1718 charge-transfer (CT) complex19 Ground-state adduct that exhibits an electronic absorption corresponding to light-20 induced transfer of electronic charge from one region of the adduct to another.21 See [9].22 rev[3]2324 chelation25 Formation or presence of bonds (or other attractive interactions) between a single 26 central atom (or ion) and two or more separate binding sites within the same ligand.27 Note 1: A molecular entity in which there is chelation (and the corresponding 28 chemical species) is called a chelate, while the species that binds to the central atom 29 is called a chelant.30 Note 2: The terms bidentate, tridentate, ... multidentate are used to indicate the 31 number of potential binding sites of the ligand, at least two of which must be used by 32 the ligand in forming a chelate. For example, the bidentate ethylenediamine forms a 33 chelate with Cu+2 in which both nitrogen atoms of ethylenediamine are bonded to 34 copper.35 Note 3: The use of the term is often restricted to metallic central atoms or ions.36 Note 4: The phrase "separate binding sites" is intended to exclude cases such as 37 [PtCl3(CH2=CH2)]–, ferrocene, and (benzene)tricarbonylchromium, in which ethene, the 38 cyclopentadienyl group, and benzene, respectively, are considered to present single 39 binding sites to the respective metal atom.40 See also ambident, cryptand.

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1 See also [29].2 [3]34 cheletropic reaction5 Cycloaddition across the terminal atoms of a π system with formation of two new 6 bonds to a single atom of a monocentric reagent. There is formal loss of one bond in 7 the substrate and an increase in coordination number of the relevant atom of the 8 reagent.9 Example: addition of sulfur dioxide to butadiene:

1011 Note: The reverse of this type of reaction is designated "cheletropic elimination". 12 See [107].13 [3]1415 chelotropic reaction16 Alternative (and etymologically more correct) name for cheletropic reaction.17 See [51].18 [3]1920 chemical flux, 21 Unidirectional rate of reaction, applicable to the progress of component reaction steps 22 in a complex system or to the progress of reactions in a system at dynamic equilibrium 23 (in which there are no observable concentration changes with time), excluding the 24 reverse reaction and other reaction steps.25 Note 1: Chemical flux is a derivative with respect to time, and has the dimensions of 26 amount of substance per volume transformed per time.27 For example, for the mechanism28 A + B C⇄29 C + D → E30 1 is the chemical flux due to forward reaction step 1, or the rate of formation of C or 31 the rate of loss of A or B due to that step, and similarly for–1 and 2.32 Note 2: The sum of all the chemical fluxes leading to C is designated the "total 33 chemical flux into C" (symbol C), and the sum of all the chemical fluxes leading to 34 destruction of C is designated the "total chemical flux out of C" (symbol –C), and 35 similarly for A and B. It then follows for this example that C = 1 and –C =–1 + 2,36 Note 3: The net rate of appearance of C is then given by

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12 d[C]/dt = C – –C 34 Note 4: In this system 1 (or C) can be regarded as the hypothetical rate of 5 formation of C due to the single (unidirectional) reaction 1 proceeding in the assumed 6 absence of all other reactions, and –C can be regarded as the hypothetical rate of 7 destruction of C due to the two (unidirectional) reactions –1 and 2.8 Note 5: Even when there is no net reaction, chemical flux can often be measured by 9 NMR methods.

10 See [1]. 11 See also order of reaction, rate-limiting step, steady state.12 rev[3]1314 chemical reaction15 Process that results in the interconversion of chemical species.16 Note 1: This definition includes experimentally observable interconversions of 17 conformers and degenerate rearrangements.18 Note 2: Chemical reactions may be elementary reactions or stepwise reactions.19 Note 3: Detectable chemical reactions normally involve sets of molecular entities, as 20 indicated by this definition, but it is often conceptually convenient to use the term also 21 for changes involving single molecular entities (i.e., "microscopic chemical events"), 22 whose reactions can now be observed experimentally.23 See also identity reaction.24 rev[3]2526 chemical relaxation27 Passage of a perturbed system toward or into chemical equilibrium.28 Note 1: A chemical reaction at equilibrium can be disturbed from equilibrium by a 29 sudden change of some external parameter such as temperature, pressure, or electric-30 field strength.31 Note 2: In many cases, and in particular when the displacement from equilibrium is 32 slight, the progress of the system towards equilibrium can be expressed as a first-order 33 process3435 c(t) – (ceq)2 = [(ceq)1 – (ceq)2] exp(–t/)3637 where (ceq)1 and (ceq)2 are the equilibrium amount concentrations of one of the chemical 38 species before and after the change in the external parameter, and c(t) is its amount 39 concentration at time t. The time parameter, called relaxation time, is related to the

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1 rate coefficients of the chemical reaction involved. Such measurements are commonly 2 used to follow the kinetics of very fast reactions.3 Note 3: Relaxation, or the passage toward equilibrium, is more general than chemical 4 relaxation, and includes relaxation of nuclear spins.5 See [108,109].6 See relaxation.7 rev[3]89 chemical shift (NMR)

10 11 Variation of the resonance frequency of a nucleus in nuclear magnetic resonance 12 (NMR) spectrometry as a consequence of its environment.13 Note 1: The chemical shift of a nucleus X, X, expressed as its frequency, X, relative 14 to that of a standard, ref, and defined as1516 X = (X – ref)/ref

1718 For 1H and 13C NMR the reference signal is usually that of tetramethylsilane (SiMe4). 19 Note 2: Chemical shift is usually reported in "parts per million" or ppm, where the 20 numerator has unit Hz, and the denominator has unit MHz, like the spectrometer's 21 operating frequency.22 Note 3: For historical reasons that predate Fourier-transform NMR, if a resonance 23 signal occurs at higher frequency than a reference signal, it is said to be downfield, and 24 if resonance occurs at lower frequency, the signal is upfield. Resonances downfield 25 from SiMe4 have positive -values, and resonances upfield from SiMe4 have negative 26 -values. These terms have been superseded, and deshielded and shielded are 27 preferred for downfield and upfield, respectively.28 See [110].29 See also shielding.30 rev[3]3132 chemical species33 Ensemble of chemically identical molecular entities that can explore the same set of 34 molecular energy levels on the time scale of an experiment. The term is applied equally 35 to a set of chemically identical atomic or molecular structural units in a solid array.36 Note 1: For example, conformational isomers may be interconverted sufficiently 37 slowly to be detectable by separate NMR spectra and hence to be considered to be 38 separate chemical species on a time scale governed by the radiofrequency of the 39 spectrometer used. On the other hand, in a slow chemical reaction the same mixture of 40 conformers may behave as a single chemical species, i.e., there is virtually complete

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1 equilibrium population of the total set of molecular energy levels belonging to the two 2 conformers.3 Note 2: Except where the context requires otherwise, the term is taken to refer to a 4 set of molecular entities containing isotopes in their natural abundance.5 Note 3: The definition given is intended to embrace not only cases such as graphite 6 and sodium chloride but also a surface oxide, where the basic structural units may not 7 be capable of isolated existence.8 Note 4: In common chemical usage, and in this Glossary, generic and specific 9 chemical names (such as radical or hydroxide ion) or chemical formulae refer either to

10 a chemical species or to a molecular entity.11 [3]1213 chemically induced dynamic nuclear polarization (CIDNP)14 Non-Boltzmann nuclear spin-state distribution produced in thermal or photochemical 15 reactions, usually from colligation and diffusion or disproportionation of radical pairs, 16 and detected by NMR spectroscopy as enhanced absorption or emission signals.17 See [111,112].18 rev[3]1920 chemiexcitation21 Generation, by a chemical reaction, of an electronically excited molecular entity from 22 reactants in their ground electronic states.23 See [9].24 [3]2526 chemoselectivity27 Feature of a chemical reagent that reacts preferentially with one of two or more different 28 functional groups.29 Note 1: A reagent has a high chemoselectivity if reaction occurs with only a limited 30 number of different functional groups. For example, sodium tetrahydridoborate (NaBH4) 31 is a more chemoselective reducing agent than is lithium tetrahydridoaluminate (LiAlH4). 32 The concept has not been defined in more quantitative terms.33 Note 2: The term is also applied to reacting molecules or intermediates that exhibit 34 selectivity towards chemically different reagents.35 Note 3: Usage of the term chemospecificity for 100 % chemoselectivity is 36 discouraged.37 See [113].38 See also regioselectivity, stereoselectivity, stereospecificity.39 [3]40

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1 chemospecificity2 obsolete3 See chemoselectivity.4 [3]56 chirality7 Property of a structure that is not superimposable on its mirror image.8 See [11,46].9 [3]

1011 chirality centre12 chiral centre (superseded)13 Atom with attached groups such that the arrangement is not superimposable on its 14 mirror image.15 Note: Often this is a tetrahedral atom with four different groups attached, such as 16 CHBrClF, or C2 of CH3CHBrCH2CH3, or the sulfur of CH3S(=O)Ph, where the lone pair 17 is considered as a fourth group.18 See [11].19 See also stereogenic centre.20 [3]2122 chiral feature23 Structural characteristic rendering a molecule chiral.24 Examples: four different substituents on a carbon atom (chirality centre), 25 conformational helix, chiral axis (as in allenes XCH=C=CHX).2627 chiral recognition28 Attraction between molecules through noncovalent interactions that exhibit 29 complementarity only between partners with specific chirality.30 See also molecular recognition.3132 chromophore33 Part (atom or group of atoms) of a molecular entity in which the electronic transition 34 responsible for a given spectral band is approximately localized.35 Note 1: The term arose originally to refer to the groupings that are responsible for a 36 dye's colour.37 Note 2: The electronic transition can often be assigned as involving n, , *, , and/or 38 * orbitals whose energy difference falls within the range of the visible or UV spectrum.39 Note 3: The term has been extended to vibrational transitions in the infrared.40 See [9,114,115].

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1 [3]23 CIDNP4 Acronym for chemically induced dynamic nuclear polarization.5 [3]67 cine-substitution8 Substitution reaction (generally aromatic) in which the entering group takes up a 9 position adjacent to that occupied by the leaving group.

10 Example:11

121314 See also tele-substitution. 15 [3]1617 classical carbocation18 Carbocation whose electronic structure can be adequately described by two-electron-19 two-centre bonds, i.e., synonymous with carbenium ion.2021 clathrate22 See host, inclusion compound.23 [3]2425 coarctate26 Feature of a concerted transformation in which the primary changes in bonding occur 27 within a cyclic array of atoms but in which two nonbonding atomic orbitals on an atom 28 interchange roles with two bonding orbitals.29 Example: epoxidation with dimethydioxirane30

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12 Note: Because the atomic orbitals that interchange roles are orthogonal, such a 3 reaction does not proceed through a fully conjugated transition state and is thus not a 4 pericyclic reaction. It is therefore not governed by the rules that express orbital 5 symmetry restrictions applicable to pericyclic reactions.6 See [116].7 See also pseudopericyclic.89 colligation

10 Formation of a covalent bond by combination or recombination of two radicals.11 Note: This is the reverse of unimolecular homolysis.12 Example:13 HO· + H3C· → CH3OH14 [3]1516 collision complex17 Ensemble formed by two reaction partners, where the distance between them is equal 18 to the sum of the van der Waals radii of neighbouring atoms.19 See also encounter complex.20 [3]2122 common-ion effect (on rates)23 Reduction in the rate of certain reactions of a substrate RX in solution [by a path that 24 involves a pre-equilibrium with formation of R+ (or R–) ions as reaction intermediates] 25 caused by the addition to the reaction mixture of an electrolyte solute containing the 26 "common ion" X– (or X+).27 Example: the rate of solvolysis of chlorodiphenylmethane in acetone-water is 28 reduced by the addition of salts of the common ion Cl–, which causes a decrease in the 29 steady-state concentration of the diphenylmethyl cation:3031 Ph2CHCl Ph2CH+ + Cl– (free ions, not ion pairs)⇄32 Ph2CH+ + 2 OH2 → Ph2CHOH + H3O+

33

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1 Note: This retardation due to a common ion should be distinguished from the 2 acceleration due to a salt effect of all ions.3 rev[3]45 compensation effect6 Observation that a plot of T∆rS vs. ∆rH (frequently T∆‡S vs. ‡H) for a series of reactions 7 with a range of different substituents (or other unique variable such as solvent or 8 dissolved salt), are straight lines of approximately unit slope, so that, e.g., the terms 9 ∆‡H and T∆‡S partially compensate, and ∆‡G = ∆‡H – T∆‡S shows less variation than

10 ∆‡H or T∆‡S separately.11 Note: Frequently such ∆‡S vs. ∆‡H correlations are statistical artifacts, arising if 12 entropy and enthalpy are extracted from the variation of an equilibrium constant or a 13 rate constant with temperature, so that the slope and intercept of the van't Hoff plots 14 are correlated. The problem is avoided if the equilibrium constant and the enthalpy are 15 measured independently, for example by spectrophotometry and calorimetry, 16 respectively.17 See [117,118,119,120].18 See also isoequilibrium relationship, isokinetic relationship.19 rev[3]2021 complex22 Molecular entity formed by loose association involving two or more component 23 molecular entities (ionic or uncharged), or the corresponding chemical species. The 24 attraction between the components is often due to hydrogen-bonding or van der Waals 25 attraction and is normally weaker than a covalent bond.26 Note 1: The term has also been used with a variety of meanings in different contexts: 27 it is therefore best avoided when a more explicit alternative is applicable, such as adduct 28 when the association is a consequence of bond formation. 29 Note 2: In inorganic chemistry the term "coordination entity" is recommended instead 30 of "complex" [29,121].31 See also activated complex, adduct, charge transfer complex, electron-donor-32 acceptor complex, encounter complex, inclusion complex, -adduct, -adduct, 33 transition state.34 [3]3536 composite reaction37 Chemical reaction for which the expression for the rate of disappearance of a reactant 38 (or rate of appearance of a product) involves rate constants of more than a single 39 elementary reaction.

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1 Examples: "opposing reactions" (where rate constants of two opposed chemical 2 reactions are involved), "parallel reactions" (for which the rate of disappearance of any 3 reactant is governed by the rate constants relating to several simultaneous reactions 4 that form different products from a single set of reactants), and stepwise reactions.5 [3]67 comproportionation8 Any chemical reaction of the type A' + A" → 2 A 9 Example:

10 Pb + PbO2 + 2 H2SO4 → 2 PbSO4 + 2 H2O ( in a lead battery)1112 Note: Other stoichiometries are possible, depending on the oxidation numbers of the 13 species.14 Reverse of disproportionation. The term "symproportionation" is also used.15 See [122].16 rev[3]1718 concerted19 Feature of a process in which two or more primitive changes occur within the same 20 elementary reaction. Such changes will normally be "energetically coupled".21 Note 1: The term "energetically coupled" means that the simultaneous progress of 22 the primitive changes involves a transition state of lower energy than that for their 23 successive occurrence.24 Note 2: In a concerted process the primitive changes may be synchronous or 25 asynchronous.26 See also bifunctional catalysis, potential-energy surface.27 [3]2829 condensation30 Reaction (usually stepwise) in which two or more reactants (or remote reactive sites 31 within the same molecular entity) yield a product with accompanying formation of water 32 or some other small molecule, e.g., ammonia, ethanol, acetic acid, hydrogen sulfide.33 Note 1: The mechanism of many condensation reactions has been shown to 34 comprise consecutive addition and elimination reactions, as in the base-catalysed 35 formation of (E)-but-2-enal (crotonaldehyde) from acetaldehyde, via dehydration of 3-36 hydroxybutanal (aldol). The overall reaction in this example is known as the aldol 37 condensation.38 Note 2: The term is sometimes also applied to cases where the formation of water 39 or other simple molecule does not occur, as in "benzoin condensation".40 [3]

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12 condensed formula3 Linear representation of the structure of a molecular entity in which bonds are omitted.4 Example: methyl 3-methylbutyl ether (isoamyl methyl ether, 5 (CH3)2CHCH2CH2OCH3, sometimes condensed further to (CH3)2CH[CH2]2OCH3)6 Note: This term is sometimes also called a line formula, because it can be written on 7 a single line, but the line formula explicitly shows all bonds.89 configuration (electronic)

10 Distribution of the electrons of an atom or a molecular entity over a set of one-electron 11 wavefunctions called orbitals, according to the Pauli principle.12 Note: From one configuration several states with different multiplicities may result. 13 For example, the ground electronic configuration of the oxygen molecule (O2) is14 1g

2, 1u2, 2g

2, 2u2, 1u

4, 3g2, 1g

2

15 resulting in16 3g

–, 1g, and 1g+ multiplets

17 See [8,9].18 [3]1920 configuration (molecular)21 Arrangement in space of the atoms of a molecular entity that distinguishes it from any 22 other molecular entity having the same molecular formula and connectivity and that is 23 not due to conformational differences (rotation about single bonds).24 See [11].25 [3]2627 conformations28 Different spatial arrangements of a molecular entity that can be interconverted by 29 rotation about one or more formally single bonds. 30 Note 1: Different conformations are often not considered to be stereoisomeric, 31 because interconversion is rapid. 32 Note 2: Different or equivalent spatial arrangements of ligands about a central atom, 33 such as those interconverted by pyramidal inversion (of amines) or Berry 34 pseudorotation (as of PF5) and other "polytopal rearrangements", are sometimes 35 considered conformations, but they are properly described as configurations.36 See [11].37 rev[3]3839 conformational isomers40 conformers

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12 conformer3 Conformation of a molecular entity that corresponds to a minimum on the potential-4 energy surface of that molecular entity.5 Note: The distinction between conformers and isomers is the height of the barrier for 6 interconversion. Isomers are stable on macroscopic timescales because the barrier for 7 interconversion is high, whereas a conformer cannot persist on a macroscopic 8 timescale because the interconversion between conformations is achieved rapidly.9 See [11].

101112 conjugate acid13 Brønsted acid BH+ formed on protonation (hydronation) of the base B.14 Note 1: B is called the conjugate base of the acid BH+.15 Note 2: The conjugate acid always carries one unit of positive charge more than the 16 base, but the absolute charges of the species are immaterial to the definition. For 17 example: the Brønsted acid HCl and its conjugate base Cl– constitute a conjugate acid-18 base pair, and so do NH4

+ and its conjugate base NH3.19 rev[3]2021 conjugated system, conjugation22 Molecular entity whose structure may be represented as a system of alternating multiple 23 and single (or ) bonds: e.g.,24 CH2=CH–CH=CH2 or CH2=CH–CΞN25 Note: In such systems, conjugation is the interaction of one p-orbital with another p-26 orbital (or d-orbital) across an intervening bond, including the analogous interaction 27 involving a p-orbital containing an unshared electron pair, e.g.,

2829 or the interaction across a double bond whose system does not interact with the p 30 orbitals of the conjugated system, as in31 CH2=C=C=CH2

32 See also cross-conjugation, delocalization, resonance, through-conjugation.33 [3]3435 connectivity36 Description of which atoms are bonded to which other atoms.37 Note: Connectivity is often displayed in a line formula or other structure showing 38 which atoms are bonded to which other atoms, but with minimal or no indication of bond 39 multiplicity.40 Example: The connectivity of propyne is specified by CH3CCH.

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1 rev[3]23 conrotatory4 Stereochemical feature of an electrocyclic reaction in which the substituents at the 5 interacting termini of the conjugated system rotate in the same sense (both clockwise 6 or both counterclockwise).7 See also disrotatory.8 rev[3]9

10 conservation of orbital symmetry11 An approach to understanding pericyclic reactions that focuses on a symmetry 12 element (e.g., a reflection plane) that is retained along a reaction pathway. If each of 13 the singly or doubly occupied orbitals of the reactant(s) is of the same symmetry as a 14 similarly occupied orbital of the product(s), that pathway is "allowed" by orbital 15 symmetry conservation. If instead a singly or doubly occupied orbital of the reactant(s) 16 is of the same symmetry as an unoccupied orbital of the product(s), and an unoccupied 17 orbital of the reactant(s) is of the same symmetry as a singly or doubly occupied orbital 18 of the product(s), that pathway is "forbidden" by orbital symmetry conservation.19 Note 1: This principle permits the qualitative construction of correlation diagrams to 20 show how molecular orbitals transform and how their energies change during chemical 21 reactions.22 Note 2: Considerations of orbital symmetry are frequently grossly simplified in that, 23 for example, the and * orbitals of a carbonyl group in an asymmetric molecule are 24 treated as having the same topology (pattern of local nodal planes) as those of a 25 symmetric molecule (e.g., CH2=O), despite the absence of formal symmetry elements.26 See [107,123].27 See also orbital symmetry.2829 constitutional isomers30 Species (or molecular entities) with the same atomic composition (molecular formula) 31 but with different connectivity.32 Note: The term structural isomers is discouraged, because all isomers differ in 33 structure and because isomers may be constitutional, configurational, or 34 conformational.35 See [11].3637 contributing structure38 resonance form39 rev[3]40

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1 coordinate covalent bond2 dative bond34 coordination5 Formation of a covalent bond, the two shared electrons of which come from only one 6 of the two partners linked by the bond, as in the reaction of a Lewis acid and a Lewis 7 base to form a Lewis adduct; alternatively, the bonding formed in this way.8 Note: In the former sense, it is the reverse of unimolecular heterolysis. 9 See also dative bond, -adduct.

10 rev[3]1112 coordination number13 Number of other atoms directly linked to a specified atom in a chemical species 14 regardless of the number of electrons in the bonds linking them [29, Rule IR-10.2.5]. 15 For example, the coordination number of carbon in methane or of phosphorus in 16 triphenylphosphane oxide (triphenylphosphine oxide) is four whereas the coordination 17 number of phosphor is five in phosphorus pentafluoride.18 Note: The term is used in a different sense in the crystallographic description of ionic 19 crystals.20 rev[3]2122 coronate23 See crown ether.24 rev[3]2526 correlation analysis27 Use of empirical correlations relating one body of experimental data to another, with the 28 objective of finding quantitative relationships among the factors underlying the 29 phenomena involved. Correlation analysis in organic chemistry often uses linear Gibbs-30 energy relations (formerly linear free-energy relation, LFER) for rates or equilibria of 31 reactions, but the term also embraces similar analysis of physical (most commonly 32 spectroscopic) properties and of biological activity.33 See [124,125,126,127,128].34 See also linear free-energy relation, quantitative structure-activity relationship 35 (QSAR).36 [3]3738 coupling constant (spin-spin coupling constant)39 J40 (Unit: Hz)

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1 Quantitative measure of nuclear spin-spin coupling in nuclear magnetic resonance 2 spectroscopy.3 Note: Spin-spin coupling constants have been correlated with atomic hybridization 4 and with molecular conformations.5 See [129,130,131].6 rev[3]78 covalent bond9 Stabilizing interaction associated with the sharing of electron pairs between two atomic

10 centres of a molecular entity, leading to a characteristic internuclear distance.11 See also agostic, coordination, hydrogen bond, multi-centre bond.12 See [8].13 rev[3]1415 Cox–Yates equation16 Generalization of the Bunnett-Olsen equation of the form1718 lg([SH+])/[S]) – lg{[H+]} = m*X + pKSH+

1920 where [H+] is the amount concentration of acid, X is the activity-coefficient ratio 21 lg({s{H+}/{SH+}) for an arbitrary reference base, pKSH+ is the thermodynamic 22 dissociation constant of SH+, and m* is an empirical parameter derived from linear 23 regression of the left-hand side vs. X. Arguments in the lg functions should be unitless. 24 Thus, the reduced quantities should be used: {[H+]} is [H+]/units.25 Note: The function X is called excess acidity because it gives a measure of the 26 difference between the acidity of a solution and that of an ideal solution of the same 27 concentration. In practice X = – (Ho + lg{[H+]}) and m* = 1 – , where Ho is the Hammett 28 acidity function and is the slope in the Bunnett-Olsen equation.29 See [132,133,134].30 See Bunnett-Olsen equation.31 rev[3]3233 critical micellisation concentration (cmc) 34 critical micelle concentration35 Relatively small range of concentrations separating the limit below which virtually no 36 micelles are detected and the limit above which virtually all additional surfactant 37 molecules form micelles.38 Note 1: Many physical properties of surfactant solutions, such as conductivity or light 39 scattering, show an abrupt change at a particular concentration of the surfactant, which 40 can be taken as the cmc.

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1 Note 2: As values obtained using different properties are not quite identical, the 2 method by which the cmc is determined should be clearly stated.3 See [135].4 See also inverted micelle.5 rev[3]67 cross-conjugation8 Phenomenon of three conjugated groups, two pairs of which exhibit conjugation but 9 without through-conjugation of all three, as in 2-phenylallyl, benzoate anion, divinyl

10 ether, or m-xylylene [1,3-C6H4(CH2)2].11 See [64].12 rev[3]1314 crown ether15 Molecular entity comprising a monocyclic ligand assembly that contains three or more 16 binding sites held together by covalent bonds and capable of binding a guest in a central 17 (or nearly central) position. The adducts formed are sometimes known as "coronates". 18 The best known members of this group are macrocyclic polyethers, such as 18-crown-19 6, containing several repeating units –CR2–CR2O– (where R is most commonly H).

2021 See [136,137].22 See also host.23 rev[3]2425 cryptand26 Molecular entity comprising a cyclic or polycyclic assembly of binding sites that contains 27 three or more binding sites held together by covalent bonds, and which defines a 28 molecular cavity in such a way as to bind (and thus "hide" in the cavity) another 29 molecular entity, the guest (a cation, an anion or a neutral species), more strongly than 30 do the separate parts of the assembly (at the same total concentration of binding sites).31 Note 1: The adduct thus formed is called a "cryptate". The term is usually restricted 32 to bicyclic or oligocyclic molecular entities.

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1 Example:

23 Note 2: Corresponding monocyclic ligand assemblies (crown ether) are sometimes 4 included in this group, if they can be considered to define a cavity in which a guest can 5 hide. The terms "podand" and "spherand" are used for certain specific ligand 6 assemblies. Coplanar cyclic polydentate ligands, such as porphyrins, are not normally 7 regarded as cryptands.8 See [138].9 See also host. See also [139].

10 [3]1112 curly arrows13 Symbols for depicting the flow of electrons in a chemical reaction or to generate 14 additional resonance forms.15 Note 1: The tail of the curly arrow shows where an electron pair originates, and the 16 head of the curly arrow shows where the electron pair goes.17 Note 2: Single-headed curly arrows are used to depict the flow of unpaired electrons.18 Examples:19

202122 See electron pushing.2324 Curtin-Hammett principle25 Statement that in a chemical reaction that yields one product (X) from one isomer (A') 26 and a different product (Y) from another isomer (A") (and provided these two isomers 27 are rapidly interconvertible relative to the rate of product formation, whereas the 28 products do not undergo interconversion) the product composition is not directly related 29 to the relative concentrations of the isomers in the substrate; it is controlled only by the 30 difference in standard Gibbs energies (G‡

A' – G‡A") of the respective transition states.

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1 Note 1: The product composition is given by [Y]/[X] = (kYk1)/(k–1kX) or KckY/kX, where 2 Kc is the equilibrium constant, [A"]/[A'], and where kY and kX are the respective rate 3 constants of their reactions; these parameters are usually unknown.4 Note 2: The energy diagram below represents the transformation of rapidly 5 interconverting isomers A' and A" into products X and Y.6

78

91011 Note 3: A related concept is the Winstein-Holness equation for the overall rate of 12 product formation in a system of rapidly interconverting isomers A' and A". The overall 13 rate constant k is given by1415 k = x'kX + x"kY

1617 where x' and x" are the respective mole fractions of the isomers at equilibrium [15]. 18 However, since x' and x" are simple functions of Kc, the apparent dependence of the 19 overall rate on the relative concentrations of A' and A" disappears. The overall rate 20 constant is the sum kX + KckY, which is a function of the same quantities that determine 21 the product ratio.22 See [140]. See also [15,141,142].23 rev[3]2425 cybotactic region

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1 That part of a solution in the vicinity of a solute molecule in which the ordering of the 2 solvent molecules is modified by the presence of the solute molecule. The term solvent 3 "cosphere" of the solute has also been used.4 See [143,144].5 [3]67 cyclization8 Formation of a ring compound from a chain by formation of a new bond.9 See also annulation.

10 [3]1112 cycloaddition13 Reaction in which two or more unsaturated molecules (or parts of the same molecule) 14 combine with the formation of a cyclic adduct in which there is a net reduction of the 15 bond multiplicity.16 Note 1: The following two systems of notation have been used for the more detailed 17 specification of cycloadditions, of which the second, more recent system (described 18 under (2)) is preferred:19 (1) An (i+j+...) cycloaddition is a reaction in which two or more molecules (or parts 20 of the same molecule), respectively, provide units of i, j,... linearly connected 21 atoms: these units become joined at their respective termini by new σ bonds so 22 as to form a cycle containing (i+j+...) atoms. In this notation, (a) a Diels-Alder 23 reaction is a (4+2) cycloaddition, (b) the initial reaction of ozone with an alkene is 24 a (3+2) cycloaddition, and (c) the reaction of norbornadiene below is a (2+2+2) 25 cycloaddition. (Parentheses are used to indicate the numbers of atoms, but 26 brackets are also used.)

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123 (2) The symbolism [i+j+...] for a cycloaddition identifies the numbers i, j,... of 4 electrons in the interacting units that participate in the transformation of reactants 5 to products. In this notation the reaction (a) and (b) of the preceding paragraph 6 would both be described as [2+4] cycloadditions, and (c) as a [2+2+2] 7 cycloaddition. The symbol a or s (a = antarafacial, s = suprafacial) is often added 8 (usually as a subscript after the number to designate the stereochemistry of 9 addition to each fragment. A subscript specifying the orbitals, viz., , (with their

10 usual significance) or n (for an orbital associated with a single atom), may be 11 added as a subscript before the number. Thus the normal Diels-Alder reaction is 12 a [4s+2s] or [4s + 2s] cycloaddition, whilst the reaction13

1415 would be a [14a+2s] or [14a + 2s] cycloaddition, leading to the stereoisomer shown, 16 with hydrogens anti. (Brackets are used to indicate the numbers of electrons, and 17 they are also used instead of parentheses to denote the numbers of atoms.)

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1 Note 2: Cycloadditions may be pericyclic reactions or (non-concerted) stepwise 2 reactions. The term "dipolar cycloaddition" is used for cycloadditions of 1,3-dipolar 3 compounds.4 See [107,145,146]. 5 See also cheletropic reaction, ene reaction, pericyclic reaction.6 rev[3]78 cycloelimination9 Reverse of cycloaddition. The term is preferred to the synonyms "cycloreversion",

10 "retro-addition", and "retrocycloaddition".11 [3]1213 cycloreversion14 obsolete15 See cycloelimination.16 [3]1718 dative bond19 obsolescent20 Coordination bond formed between two chemical species, one of which serves as a 21 donor and the other as an acceptor of the electron pair that is shared in the bond.22 Examples: the N–B bond in H3N+–B–H3, the S–O bond in (CH3)2S+–O–.23 Note 1: A distinctive feature of dative bonds is that their minimum-energy rupture in 24 the gas phase or in inert solvent follows the heterolytic bond-cleavage path.25 Note 2: The term is obsolescent because the distinction between dative bonds and 26 ordinary covalent bonds is not useful, in that the precursors of the bond are irrelevant: 27 H3N+–B–H3 is the same molecule, with the same bonds, regardless of whether the 28 precursors are considered to have been H3N + BH3 or H3N·+ + –BH3.29 See [8].30 See coordination.31 rev[3]3233 degenerate chemical reaction34 See identity reaction.35 [3]3637 degenerate rearrangement38 Chemical reaction in which the product is indistinguishable (in the absence of isotopic 39 labelling) from the reactant.

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1 Note 1: The term includes both "degenerate intramolecular rearrangements" and 2 reactions that involve intermolecular transfer of atoms or groups ("degenerate 3 intermolecular reactions"): both are degenerate isomerizations.4 Note 2: The occurrence of degenerate rearrangements may be detectable by isotopic 5 labelling or by dynamic NMR techniques. For example: the [3,3]sigmatropic 6 rearrangement of hexa-1,5-diene (Cope rearrangement),7 Note 3: Synonymous but less preferable terms are "automerization", "permutational 8 isomerism", "isodynamic transformation", "topomerization".9 See [147].

10 See also fluxional, molecular rearrangement, narcissistic reaction, valence isomer.11 [3]1213 delocalization14 Quantum-mechanical concept most usually applied in organic chemistry to describe the 15 redistribution of electrons in a conjugated system, where each link has a fractional 16 double-bond character, or a non-integer bond order, rather than electrons that are 17 localized in double or triple bonds.18 Note 1: There is a corresponding "delocalization energy", identifiable with the 19 stabilization of the system relative to a hypothetical alternative in which formal

20 (localized) single and double bonds are present. Some degree of delocalization is 21 always present and can be estimated by quantum mechanical calculations. The effects 22 are particularly evident in aromatic systems and in symmetrical molecular entities in 23 which a lone pair of electrons or a vacant p-orbital is conjugated with a double bond 24 (e.g., carboxylate ions, nitro compounds, enamines, the allyl cation).25 Note 2: Delocalization in such species may be represented by partial bonds or by 26 resonance (here symbolized by a two-headed arrow) between resonance forms.27

28

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1 These examples also illustrate the concomitant delocalization of charge in ionic 2 conjugated systems. Analogously, delocalization of the spin of an unpaired electron 3 occurs in conjugated radicals.4 Note 3: Delocalization is not limited to electrons. Hyperconjugation is the 5 delocalization of electrons of bonds.6 [3]78 dendrimer 9 Large molecule constructed from a central core with repetitive branching and multiple

10 functional groups at the periphery. 11 See [148].12 Example: (with 48 CH2OH at the periphery) 13

141516 Note: The name comes from the Greek (dendron), which translates to "tree". 17 Synonymous terms include arborols and cascade molecules.18 See [148,149,150,151,152].1920 deshielding21 See shielding.22 [3]2324 detachment25 Reverse of attachment.26 [3]272829

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1 detailed balancing2 Principle that when equilibrium is reached in a reaction system (containing an arbitrary 3 number of components and reaction paths), as many atoms, in their respective 4 molecular entities, will pass forward in a given finite time interval as will pass backward 5 along each individual path.6 Note 1: It then follows that the reaction path in the reverse direction must in every 7 detail be the reverse of the reaction path in the forward direction (provided that the 8 system is at equilibrium).9 Note 2: The principle of detailed balancing is a consequence for macroscopic

10 systems of the principle of microscopic reversibility.11 [3]1213 diamagnetism14 Property of substances having a negative magnetic susceptibility (χ), whereby they are 15 repelled out of a magnetic field.16 See also paramagnetism.17 [3]1819 diastereoisomerism20 Stereoisomerism other than enantiomerism.21 See diastereoisomers [11].22 [3]2324 diastereomeric excess (diastereoisomeric excess)25 x1 – x2, where x1 and x2 (with x1 + x2 = 1) are the mole fractions of two diastereoisomers 26 in a mixture, or the fractional yields of two diastereoisomers formed in a reaction.27 Note: Frequently this term is abbreviated to d.e.28 See stereoselectivity, diastereoisomers.29 See [11].3031 diastereomeric ratio32 x1/x2, where x1 and x2 are the mole fractions of two diastereoisomers in a mixture formed 33 in a reaction.34 Note: Frequently this term is abbreviated to d.r.35 See stereoselectivity, diastereoisomers.36 See [11].3738 diastereoisomers (diastereomers)39 Stereoisomers not related as mirror images of each other.

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1 Note: Diastereoisomers are characterized by differences in physical properties, and 2 by differences in chemical behaviour toward chiral as well as achiral reagents.3 See [11].45 diastereoselectivity6 Preferential formation in a chemical reaction of one diastereoisomer over another.7 Note: This can be expressed quantitatively by the diastereoisomeric excess or by the 8 diastereomeric ratio, which is preferable because it is more closely related to a Gibbs-9 energy difference.

10 See [11].11 See selectivity.1213 dielectric constant14 obsolete15 See [12].16 See permittivity (relative).17 rev[3]1819 dienophile20 Ene or yne component of a Diels-Alder reaction, including compounds with hetero-21 double bonds and hetero-triple bonds.22 See cycloaddition.23 rev[3]2425 diffusion-controlled rate26 See encounter-controlled rate, microscopic diffusion control. Contrast mixing control.27 [3]2829 dimerization30 Transformation of a molecular entity A to give a molecular entity A2.31 Examples:32 2 CH3· → CH3CH3

33 2 CH3COCH3 → (CH3)2C(OH)CH2COCH3

34 2 RCOOH → (RCOOH)2

35 See also association.36 [3]3738 Dimroth-Reichardt ET parameter

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1 Quantitative measure of solvent polarity, based on the wavelength, max, of the longest-2 wavelength intramolecular charge-transfer absorption band of the solvatochromic 3 betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridin-1-ium-1-yl)phenolate.4

56 See [17,153,154,155].7 See solvent parameter.8 rev[3]9

10 dipolar aprotic solvent11 See dipolar non-HBD solvent.12 rev[3]1314 dipolar non-HBD solvent (Non-hydrogen-bond donating solvent)15 Solvent with a comparatively high relative permittivity ("dielectric constant"), greater 16 than ca. 15, and composed of molecules that have a sizable permanent dipole moment 17 and that, although it may contain hydrogen atoms, cannot donate suitably labile 18 hydrogen atoms to form strong solvent-solute hydrogen bonds.19 Examples: dimethyl sulfoxide, acetonitrile, acetone, as contrasted with methanol and 20 N-methylformamide.21 Note 1: The term “dipolar” refers to solvents whose molecules have a permanent 22 dipole moment, in contrast to solvents whose molecules have no permanent dipole 23 moment and should be termed “apolar” or "nonpolar".24 Note 2: Non-HBD solvents are often called aprotic, but this term is misleading 25 because a proton can be removed by a sufficiently strong base. The aprotic nature of 26 a solvent molecule means that its hydrogens are in only covalent C–H bonds and not 27 in polar O–H+ or N–H+ bonds that can serve as hydrogen-bond donors. Use of 28 “aprotic” is therefore discouraged, unless the context makes the term unambiguous.29 Note 3: It is recommended to classify solvents according to their capability to donate 30 or not donate, as well as to accept or not accept, hydrogen bonds to or from the solute, 31 as follows:32 Hydrogen-bond donating solvents (short: HBD solvents), formerly protic solvents33 Non-hydrogen-bond donating solvents (short: non-HBD solvents), formerly aprotic 34 solvents35 Hydrogen-bond accepting solvents (short: HBA solvents)

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1 Non-hydrogen-bond accepting solvents (short: non-HBA solvents)2 See [156,157].34 dipole-dipole excitation transfer5 Förster resonance-energy transfer (FRET)6 See [9].78 dipole-dipole interaction9 Intermolecular or intramolecular interaction between molecules or groups having a

10 permanent electric dipole moment. The strength of the interaction depends on the 11 distance and relative orientation of the dipoles.12 Note: A dipole/dipole interaction is a simplification of the electrostatic interactions 13 between molecules that originate from asymmetries in the electron densities. Such 14 interactions can be described more correctly by the use of higher-order multipole 15 moments.16 See also van der Waals forces.17 rev[3]1819 dipole-induced dipole forces20 See van der Waals forces.21 [3]2223 diradical24 biradical 25 Even-electron molecular entity with two (possibly delocalized) radical centres that act 26 nearly independently of each other, e.g.,

2728 Note 1: Species in which the two radical centres interact significantly are often 29 referred to as "diradicaloids". If the two radical centres are located on the same atom, 30 the species is more properly referred to by its generic name: carbene, nitrene, etc.31 Note 2: The lowest-energy triplet state of a diradical lies below or at most only a little 32 above its lowest singlet state (usually judged relative to kBT, the product of the 33 Boltzmann constant kB and the absolute temperature T). If the two radical centres 34 interact significantly, the singlet state may be more stable. The states of those diradicals 35 whose radical centres interact particularly weakly are most easily understood in terms 36 of a pair of local doublets.

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1 Note 3: Theoretical descriptions of low-energy states of diradicals display two 2 unsaturated valences: the dominant valence-bond structures have two dots, the low-3 energy molecular orbital configurations have only two electrons in two approximately 4 nonbonding molecular orbitals, and two of the natural orbitals have occupancies close 5 to one.6 See [9,158,159,160,161,162,163].7 See also carbene, nitrene.8 [3]9

10 dispersion forces11 See London forces, van der Waals forces.12 [3]1314 disproportionation15 Any chemical reaction of the type A + A ⇋ A' + A", where A, A' and A" are different 16 chemical species. 17 Examples:18 2 R2COH· (ketyl radical) → R2C=O + R2CHOH19 Cannizzaro reaction: 2 ArCH=O → ArCH2OH + ArCOOH2021 Note 1: The reverse of disproportionation is called comproportionation.22 Note 2: A special case of disproportionation (or "dismutation") is "radical 23 disproportionation", exemplified by2425 ·CH2CH3 + ·CH2CH3 → CH2=CH2 + CH3CH3

2627 Note 3: A somewhat more restricted usage of the term prevails in inorganic 28 chemistry, where A, A' and A" are of different oxidation states.29 [3]3031 disrotatory32 Stereochemical feature of an electrocyclic reaction in which the substituents at the 33 interacting termini of the conjugated system rotate in opposite senses (one clockwise 34 and the other counterclockwise).35 See also conrotatory.36 rev[3]3738 dissociation39 Separation of a molecular entity into two or more molecular entities (or any similar 40 separation within a polyatomic molecular entity).

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1 Example:2 NH4

+(aq) → H3O+ + NH3 or CH3COOH(aq) → H3O+ + CH3CO2

–

3 Note 1: Although the separation of the constituents of an ion pair into free ions is a 4 dissociation, the ionization that produces the ion pair is not a dissociation, because the 5 ion pair is a single molecular entity.6 Note 2: The reverse of dissociation is association.7 rev[3]89 distonic ion

10 Radical ion in which charge and radical sites are separated.11 Example: •CH2CH2OCH2

+

12 See [164,165].13 rev[3]1415 distortion interaction model (Activation Strain Model)16 Method for analyzing activation energy as the sum of the energies to distort the 17 reactants into the geometries they have in transition states plus the energy of interaction 18 between the two distorted reactants.19 See [166].2021 distribution ratio22 partition ratio23 Ratio of concentrations of a solute in a mixture of two immiscible phases at equilibrium.24 See also Hansch constant.25 See [167].2627 di--methane rearrangement28 Photochemical reaction of a molecular entity with two -systems separated by a 29 saturated carbon, to form an ene-substituted cyclopropane.30 Pattern:

3132 See [9,168].3334 donicity (also called donor number, DN, which is a misnomer)35 Quantitative experimental measure of the Lewis basicity of a molecule B, expressed as 36 the negative enthalpy of the formation of the 1:1 complex B·SbCl5.37 Note: donicity is often used as a measure of a solvent’s basicity.

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1 See also acceptor parameter, Lewis basicity.2 See [63,169,170].34 downfield 5 superseded but still widely used in NMR to mean deshielded.6 See chemical shift, shielding.7 rev[3]89 driving force

10 (1) Negative of the Gibbs energy change (∆rG0) on going from the reactants to the 11 products of a chemical reaction under standard conditions. Also called affinity.12 (2) Qualitative term that relates the favorable thermodynamics of a reaction to a specific 13 feature of molecular structure, such as the conversion of weaker bonds into stronger 14 (CH3–H + Br–Br → CH3–Br + H–Br), neutralization of an acid (Claisen condensation of 15 2 CH3COOEt + EtO– to CH3COCH–COOEt + EtOH), or increase of entropy 16 (cycloelimination of cyclohexene to butadiene + ethylene).17 Note 1: This term is a misnomer, because favorable thermodynamics is due to 18 energy, not force.19 Note 2: This term has also been used in connection with photoinduced electron 20 transfer reactions, to indicate the negative of the estimated standard Gibbs energy 21 change for the outer sphere electron transfer (∆ETG0) [9].22 rev[3]2324 dual substituent-parameter equation25 Any equation that expresses substituent effects in terms of two parameters.26 Note: In practice the term is used specifically for an equation for modeling the effects 27 of meta- and para-substituents X on chemical reactivity, spectroscopic properties, etc. 28 of a probe site in benzene or other aromatic system.29 PX = I I + R R

30 where PX is the magnitude of the property for substituent X, expressed relative to the 31 property for X = H; I and R are inductive (or polar) and resonance substituent 32 constants, respectively, there being various scales for R; I and R are the 33 corresponding regression coefficients.34 See [171,172,173]. 35 See also extended Hammett equation, Yukawa-Tsuno equation.36 [3]3738 dynamic NMR39 NMR spectroscopy of samples undergoing chemical reactions.

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1 Note: Customarily this does not apply to samples where the composition of the 2 sample, and thus its NMR spectrum, changes with time, but rather to samples at 3 equilibrium, without any net reaction. The occurrence of chemical reactions is 4 manifested by features of the NMR line-shape or by magnetization transfer.5 See chemical flux.67 dyotropic rearrangement8 Process in which two bonds simultaneously migrate intramolecularly.9 Example

1011 See [174,175,176].12 rev[3]1314 educt15 deprecated: usage strongly discouraged16 starting material, reactant17 Note: This term should be avoided and replaced by reactant, because it means 18 "something that comes out", not "something that goes in".19 rev[3]2021 effective charge, Zeff

22 Net positive charge experienced by an electron in a polyelectronic atom, which is less 23 than the full nuclear charge because of shielding by the other electrons.24 rev[3]2526 effective molarity (effective concentration)27 Ratio of the first-order rate constant or equilibrium constant of an intramolecular 28 reaction involving two functional groups within the same molecular entity to the second-29 order rate constant or equilibrium constant of an analogous intermolecular elementary 30 reaction.31 Note: This ratio has unit of concentration, mol dm–3 or mol L–1, sometimes denoted 32 by M.33 See [177].34 See also intramolecular catalysis.35 [3]3637

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1 eighteen-electron rule2 Electron-counting rule that the number of nonbonding electrons at a metal plus the 3 number of electrons in the metal-ligand bonds should be 18.4 Note: The 18-electron rule in transition-metal chemistry is an analogue of the Lewis 5 octet rule.6 [3]78 electrocyclic reaction (electrocyclization)9 Molecular rearrangement that involves the formation of a bond between the termini

10 of a fully conjugated linear -electron system (or a linear fragment of a -electron 11 system) and a decrease by one in the number of π bonds, or the reverse of that process.12 Examples:

1314 Note: The stereochemistry of such a process is termed "conrotatory" if the 15 substituents at the interacting termini of the conjugated system both rotate in the same 16 sense, as in

1718 or "disrotatory" if one terminus rotates in a clockwise and the other in a 19 counterclockwise sense, as in

2021 See also pericyclic reaction.22 [3]2324 electrofuge25 Leaving group that does not carry away the bonding electron pair.

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1 Examples: In the nitration of benzene by NO2+, H+ is the electrofuge, and in an SN1

2 reaction the carbocation is the electrofuge.3 Note 1: Electrofugality characterizes the relative rates of atoms or groups to depart 4 without the bonding electron pair. Electrofugality depends on the nature of the reference 5 reaction and is not the reverse of electrophilicity [178].6 Note 2: For electrofuges in SN1 reactions see [179].7 See also electrophile, nucleofuge.8 rev[3]9

10 electromeric effect11 obsolete12 rev[3]1314 electron acceptor15 Molecular entity to which an electron may be transferred.16 Examples: 1,4-dinitrobenzene,1,1'-dimethyl-4,4'-bipyridinium dication, 17 benzophenone.18 Note 1: A group that accepts electron density from another group is not called an 19 electron acceptor but an electron-withdrawing group.20 Note 2: A Lewis acid is not called an electron acceptor but an electron-pair acceptor.21 rev[3]2223 electron affinity24 Energy released when an additional electron (without excess energy) attaches itself to 25 a molecular entity (often electrically neutral but not necessarily).26 Note 1: Equivalent to the minimum energy required to detach an electron from a 27 singly charged negative ion.28 Note 2: Measurement of electron affinities is possible only in the gas phase, but there 29 are indirect methods for evaluating them from solution data, such as polarographic half-30 wave potentials or charge-transfer spectra.31 See [8,180,181].32 rev[3]3334 electron capture35 Transfer of an electron to a molecular entity, resulting in a molecular entity of 36 (algebraically) increased negative charge.3738 electron-deficient bond39 Bond between adjacent atoms that is formed by fewer than two electrons.40 Example:

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12 Note: The B–H–B bonds are also called "two-electron three-centre bonds".3 [3]45 electron density6 The electron density at a point with coordinates x,y,z in an atom or molecular entity is 7 the product of the probability P(x,y,z) (units: m-3) of finding an electron at that point with 8 the volume element dx dy dz (units: m3).9 Note: For many purposes (e.g., X-ray scattering, forces on atoms) the system

10 behaves as if the electrons were spread out into a continuous distribution, which is a 11 manifestation of the wave-particle duality.12 See also atomic charge, charge density.13 rev[3]1415 electron donor16 Molecular entity that can transfer an electron to another molecular entity, or to the 17 corresponding chemical species.18 Note 1: After the electron transfer the two entities may separate or remain 19 associated.20 Note 2: A group that donates electron density to another group is an electron-21 donating group, regardless of whether the donation is of or electrons.22 Note 3: A Lewis base is not called an electron donor, but an electron-pair donor.23 See also electron acceptor.24 rev[3]2526 electron-donor-acceptor complex 27 obsolete28 charge-transfer complex.29 See also adduct, coordination.3031 electron-pair acceptor32 Lewis acid.33 [3]34353637 electron-pair donor38 Lewis base.39 [3]40

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1 electron pushing2 Method using curly arrows for showing the formal movement of an electron pair (from 3 a lone pair or a or bond) or of an unpaired electron, in order to generate additional 4 resonance forms or to denote a chemical reaction.5 Note 1: The electron movement may be intramolecular or intermolecular.6 Note 2: When a single electron is transferred, a single-headed curly arrow or “fish-7 hook” is used, but the electron movement in the opposite direction is redundant and 8 sometimes omitted.9 Examples:

101112 electron transfer13 Transfer of an electron from one molecular entity to another, or between two localized 14 sites in the same molecular entity.15 See also inner sphere (electron transfer), Marcus equation, outer sphere (electron 16 transfer).17 [3]1819 electron-transfer catalysis20 Process describing a sequence of reactions such as shown in equations (1)-(3), leading 21 from A to B, via species A·– and B·– that have an extra electron:22 A + e– → A·– (1)23 A·– → B·– (2)24 B·– + A → B + A·– (3)25 Note 1: An analogous sequence involving radical cations (A·+, B·+) also occurs.26 Note 2: The most notable example of electron-transfer catalysis is the SRN1 (or 27 T+DN+AN) reaction of haloarenes with nucleophiles.28 Note 3: The term has its origin in an analogy to acid-base catalysis, with the electron 29 instead of the proton. However, there is a difference between the two catalytic 30 mechanisms, since the electron is not a true catalyst, but rather behaves as the initiator 31 of a chain reaction. "Electron-transfer induced chain reaction" is a more appropriate 32 term for the mechanism described by equations (1)-(3).33 See [182,183].34 [3]3536 electronation

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1 obsolete2 See reduction.3 rev[3]45 electronegativity6 Measure of the power of an atom to attract electrons to itself.7 Note 1: The concept has been quantified by a number of authors. The first, due to 8 Pauling, is based on bond-dissociation energies, Ed (units: eV), and is anchored by 9 assigning the electronegativity of hydrogen as r,H = 2.1. For atoms A and B

1011 r,A – r,B = Ed(A–B)/eV – ½ [Ed(A–A) + Ed(B–B)]/eV}1213 where r denotes the Pauling relative electronegativity; it is relative in the senses that 14 the values are of dimension 1 and that only electronegativity differences are defined on 15 this empirical scale. See [12] section 2.5.16 Note 2: Alternatively, the electronegativity of an element, according to the Mulliken 17 scale, is the average of its atomic ionization energy and electron affinity. Other scales 18 have been developed by Allred and Rochow, by Sanderson, and by Allen.19 Note 3: Absolute electronegativity is property derived from density functional theory 20 [8].21 See [75,184,185,186,187,188,189,190].22 rev[3]2324 electronic effects (of substituents)25 Changes exerted by a substituent on a molecular property or molecular reactivity, often 26 distinguished as inductive (through-bond polarization), through-space electrostatics 27 (field effect), or resonance, but excluding steric.28 Note 1: The obsolete terms mesomeric and electromeric are discouraged.29 Note 2: Quantitative scales of substituent effects are available.30 See [126,173,191].31 See also polar effect.32 rev[3]3334 electrophile (n.), electrophilic (adj.)35 Reagent that forms a bond to its reaction partner (the nucleophile) by accepting both 36 bonding electrons from that partner.37 Note 1: Electrophilic reagents are Lewis acids.38 Note 2: "Electrophilic catalysis" is catalysis by Lewis acids.

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1 Note 3: The term "electrophilic" is also used to designate the apparent polar 2 character of certain radicals, as inferred from their higher relative reactivities with 3 reaction sites of higher electron density.4 See also electrophilicity.5 [3]67 electrophilic substitution8 Heterolytic reaction in which the entering group adds to a nucleophile and in which the 9 leaving group, or electrofuge, relinquishes both electrons to its reaction partner,

10 whereupon it becomes another potential electrophile.11 Example (azo coupling):12 RN2

+ (electrophile) + H2NC6H5 (nucleophile) → p-H2NC6H4N=NR + H+ (electrofuge)13 Note: It is arbitrary to emphasize the electrophile and ignore the feature that this is 14 also a nucleophilic substitution, but the distinction depends on the electrophilic nature 15 of the reactant that is considered to react with the substrate.16 See also substitution.1718 electrophilicity19 Relative reactivity of an electrophile toward a common nucleophile.20 Note: The concept is related to Lewis acidity. However, whereas Lewis acidity is 21 measured by relative equilibrium constants toward a common Lewis base, 22 electrophilicity is measured by relative rate constants for reactions of different 23 electrophilic reagents towards a common nucleophilic substrate.24 See [192,193,194].25 See also nucleophilicity.26 rev[3]2728 element effect29 Ratio of the rate constants of two reactions that differ only in the identity of the element 30 in the leaving group.31 Example: kBr/kCl for the reaction of N3

– (azide) with CH3Br or CH3Cl.32 rev[3]3334 elementary reaction35 Reaction for which no reaction intermediates have been detected or need to be 36 postulated in order to describe the chemical reaction on a molecular scale. An 37 elementary reaction is assumed to occur in a single step and to pass through only one 38 transition state or none.39 See [13].40 See also composite reaction, stepwise reaction.

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1 rev[3]23 elimination4 Reverse of an addition reaction.5 Note 1: In an elimination two groups (called eliminands) are lost, most often from two 6 different centres (1,2-elimination (-elimination) or 1,3-elimination, etc.) with 7 concomitant formation of an unsaturation (double bond, triple bond) in the molecule, or 8 formation of a ring.9 Note 2: If the groups are lost from a single carbon or nitrogen centre (1,1-elimination,

10 -elimination), the resulting product is a carbene or nitrene, respectively.11 rev[3]1213 empirical formula14 List of the elements in a chemical species, with integer subscripts indicating the simplest 15 possible ratios of all elements.16 Note 1: In organic chemistry C and H are listed first, then the other elements in 17 alphabetical order.18 Note 2: This differs from the molecular formula, in which the subscripts indicate how 19 many of each element is included and which is an integer multiple of the empirical 20 formula. For example, the empirical formula of glucose is CH2O while its molecular 21 formula is C6H12O6.22 Note 3: The empirical formula is the information provided by combustion analysis, 23 which has been largely superseded by mass spectrometry, which provides the 24 molecular formula.2526 enantiomer27 One of a pair of stereoisomeric molecular entities that are non-superimposable mirror 28 images of each other.29 See [11].30 [3]3132 enantiomeric excess33 Absolute value of the difference between the mole fractions of two enantiomers:3435 x(e.e.) = |x+ – x –|3637 where x+ + x– = 1.38 Note: Enantiomeric excess can be evaluated experimentally from the observed 39 specific optical rotatory power []obs, relative to []max, the (maximum) specific optical 40 rotatory power of a pure enantiomer:

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12 x(e.e.) = | []obs/[]max |34 and also by chiral chromatography, NMR, and MS methods.5 See [11].6 rev[3]78 enantiomeric ratio9 mole fraction of one enantiomer in a mixture divided by the mole fraction of the other.

1011 r(e.r.) = x+/x– or x–/x+

1213 where x+ + x– = 114 See [11].15 rev[3]1617 enantioselectivity18 See stereoselectivity.19 [3]2021 encounter complex22 Complex of molecular entities produced at an encounter-controlled rate, and which 23 occurs as an intermediate in a reaction.24 Note 1: When the complex is formed from two molecular entities it is called an 25 "encounter pair". A distinction between encounter pairs and (larger) encounter 26 complexes may be relevant for mechanisms involving pre-association.27 Note 2: The separation between the entities is small compared to the diameter of a 28 solvent molecule.29 See also [9].30 rev[3]3132 encounter-controlled rate33 Rate of reaction corresponding to the rate at which the reacting molecular entities 34 encounter each other. This is also known as the "diffusion-controlled rate", since rates 35 of encounter are themselves controlled by diffusion rates (which in turn depend on the 36 viscosity of the medium and the dimensions of the reacting molecular entities).37 Note: At 25 °C in most solvents, including water, a bimolecular reaction that proceeds 38 at an encounter-controlled rate has a second-order rate constant of 109 to 1010 dm3 39 mol–1 s–1.40 See also microscopic diffusion control.

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1 [3]23 ene reaction4 Addition of a compound with a double bond and an allylic hydrogen (the "ene") to a 5 compound with a multiple bond (the "enophile") with transfer of the allylic hydrogen and 6 a concomitant reorganization of the bonding.7 Example, with propene as the ene and ethene as the enophile.

89 Note: The reverse is a "retro-ene" reaction.

10 [3]1112 energy of activation13 Ea or EA

14 Arrhenius energy of activation15 activation energy16 (SI unit: kJ mol–1)17 Operationally defined quantity expressing the dependence of a rate constant on 18 temperature according to19

20 𝐸a(𝑇) = –𝑅 dln{

𝑘(𝑇)[𝑘] }

d(1𝑇)

2122 as derived from the Arrhenius equation, k(T) = A exp(EA/RT), where A is the pre-23 exponential factor and R the gas constant. As the argument of the ln function, k should 24 be divided by its units, i.e., by [k].25 Note 1: According to collision theory, the pre-exponential factor A is the frequency of 26 collisions with the correct orientation for reaction and Ea (or E0) is the threshold energy 27 that collisions must have for the reaction to occur.28 Note 2: The term Arrhenius activation energy is to be used only for the empirical 29 quantity as defined above. There are other empirical equations with different activation 30 energies, see [12].31 See [13].32 See also enthalpy of activation.

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1 rev[3]23 energy profile4 See Gibbs energy diagram, potential-energy profile.5 [3]67 enforced concerted mechanism8 Situation where a putative intermediate possesses a lifetime shorter than a bond 9 vibration, so that the steps become concerted.

10 See [195,196,197].11 rev[3]1213 enthalpy of activation (standard enthalpy of activation)14 ∆‡Ho

15 (SI unit: kJ mol–1)16 Standard enthalpy difference between the transition state and the ground state of the 17 reactants at the same temperature and pressure. It is related experimentally to the 18 temperature dependence of the rate coefficient k according to equation (1) for first-order 19 rate constants:20

21 ∆‡Hº = –R (1) { ∂ln (𝑘s ―1

𝑇 K ) ∂(1

𝑇) }𝑃

2223 and to equation (2) for second order rate coefficients24

25 ∆‡Hº = –R (2) {∂ln (𝑘 (mol dm3 s -1)𝑇 K )

∂(1𝑇) }

𝑃26

27 This quantity can be obtained, along with the entropy of activation ∆‡Sº, from the slope 28 and intercept of the linear least-squares fit of rate coefficients k to the equation

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1

2 ln = ∆‡Sº/R – ∆‡Hº/RT + ln[ (kB/J K-1)/(h/J s) ] (𝑘s -1

𝑇 K )3 for a first-order rate coefficient4 and

5 ln = ∆‡Sº/R – ∆‡Hº/R (1/T) + ln[ (kB/J K-1)/(h/J s) ] (𝑘 (mol dm3 s -1)𝑇 K )

67 for a second-order rate coefficient where kB is Boltzmann constant, h is Planck constant, 8 and kB/h = 2.08366… 1010 K-1 s-1.9 Note 1: An advantage of the least-squares fit is that it can also give error estimates

10 for ∆‡Hº and ∆‡Sº.11 Note 2: It is also given by1213 ∆‡Hº = RT2(∂ln k/[k]/∂T)P – RT = Ea – RT1415 ∆‡Hº = RT2(∂ln{k}//∂T)P – RT1617 where Ea is the energy of activation, provided that the rate coefficients for reactions 18 other than first-order are expressed in temperature-independent concentration units 19 (e.g., mol kg–1, measured at a fixed temperature and pressure). The argument in a 20 logarithmic function should be of dimension 1. Thus k is divided by its units, i.e., by [k], 21 as in the alternative form, in terms of reduced variables.22 See also entropy of activation, Gibbs energy of activation.23 See [12,13].24 rev[3]2526 entropy of activation, (standard entropy of activation)27 ∆‡Sº28 (SI unit: J mol–1 K–1)29 Standard entropy difference between the transition state and the ground state of the 30 reactants, at the same temperature and pressure. It is related to the Gibbs energy of 31 activation and enthalpy of activation by the equations3233 ∆‡Sº = (∆‡Hº –∆‡Gº)/T34

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1 provided that rate coefficients for reactions other than first-order reactions are 2 expressed in temperature-independent concentration units (e.g., mol dm–3, measured 3 at fixed temperature and pressure). The numerical value of S depends on the standard 4 state (and therefore on the concentration units selected).5 Note 1: It can also be obtained from the intercept of the linear least-squares fit of rate 6 coefficients k to the equation78 ln(k/[k]/T[T]) = ∆‡Sº/R – ∆‡Hº/R (1/T) + ln(kB/h),9

10 ln({k}/{T}) = ∆‡Sº/R – ∆‡Hº/R (1/T) + ln(kB/h),1112 where kB/h = 2.08366… 1010 K-1 s-1. k should be divided by its units, i.e., by [k] = s-1 13 for first-order, and by [k] = (s-1 mol-1 dm3) for second-order rate coefficients and T should 14 be divided by its units [T] = K, as in the second equation, in terms of reduced variables.. 15 Note 2: The information represented by the entropy of activation may alternatively 16 be conveyed by the pre-exponential factor A, which reflects the fraction of collisions 17 with the correct orientation for reaction (see energy of activation).18 See [12,13].19 rev[3]2021 epimer22 Diastereoisomer that has the opposite configuration at only one of two or more 23 tetrahedral stereogenic centres in the respective molecular entity.24 See [11]. 25 [3]2627 epimerization28 Interconversion of epimers by reversal of the configuration at one of the stereogenic 29 centres.30 See [11].31 [3]3233 equilibrium, chemical34 Situation in which reversible processes (processes that may be made to proceed in 35 either the forward or reverse direction by the infinitesimal change of one variable) have 36 reached a point where the rates in both directions are identical, so that the amount of 37 each species no longer changes.38 Note 1: In this situation the Gibbs energy, G, is a minimum. Also, the sum of the 39 chemical potentials of the reactants equals that of the products, so that

40 ∆rGº = –RT ln K

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1 where the thermodynamic equilibrium constant, K (of dimension 1), is the product of 2 product activities divided by the product of reactant activities.3 Note 2: In dilute solutions the numerical values of the thermodynamic activities may 4 be approximated by the respective concentrations.5 rev[3]67 equilibrium control8 See thermodynamic control.9 [3]

1011 ET-value12 See Dimroth-Reichardt ET parameter, Z-value.13 [3]1415 excess acidity16 See Bunnett-Olsen equations, Cox-Yates equation.17 [3]1819 excimer (“excited dimer”)20 Complex formed by the interaction of an excited molecular entity with another identical 21 molecular entity in its ground state.22 Note: The complex is not stable in the ground state.23 See [9].24 See also exciplex.25 [3]2627 exciplex28 Electronically excited complex of definite stoichiometry that is non-bonding in the 29 ground state.30 Note: An exciplex is a complex formed by the noncovalent interaction of an excited 31 molecular entity with the ground state of a different molecular entity, but an excimer, 32 formed from two identical components, is often also considered to be an exciplex.33 See [9].34 See also excimer.35 [3]3637 excited state38 Condition of a system with energy higher than that of the ground state. This term is 39 most commonly used to characterize a molecular entity in one of its electronically

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1 excited states, but may also refer to vibrational and/or rotational excitation in the 2 electronic ground state.3 See [9].4 [3]56 EXSY (NMR exchange spectroscopy)7 Two-dimensional NMR technique producing cross peaks corresponding to site-to site 8 chemical exchange. The cross-peak amplitudes carry information about exchange 9 rates.

10 See [198].1112 extended Hammett equation13 Multiparameter extension of the Hammett equation for the description of substituent 14 effects.15 Note 1: The major extensions using two parameters (dual substituent-parameter 16 equations) were devised for the separation of inductive and steric effects (Taft equation) 17 or of inductive (or field) and resonance effects.18 Note 2: Other parameters may be added (polarizability, hydrophobicity…) when 19 additional substituent effects are operative.20 See [191].21 See also Yukawa-Tsuno equation.22 [3]2324 external return (external ion-pair recombination)25 Recombination of free ions formed in an SN1 reaction (as distinguished from ion-pair 26 collapse).27 See ion-pair recombination.28 rev[3]2930 extrusion31 Transformation in which an atom or group Y connected to two other atoms or groups X 32 and Z is lost from a molecule, leading to a product in which X is bonded to Z, i.e.,33 X–Y–Z → X–Z + Y34 Example

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12 Note 1: When Y is a metal, this process is called a reductive elimination. 3 Note 2: The reverse of an extrusion is called an insertion.4 See also cheletropic reaction.5 [3]67 field effect8 Experimentally observable substituent effect (on reaction rates, etc.) of intramolecular 9 electrostatic interaction between the centre of interest and a monopole or dipole, by

10 direct electric-field action through space rather than through bonds.11 Note 1: The magnitude of the field effect depends on the monopole charge or dipole 12 moment, on the orientation of the dipole, on the distance between the centre of interest 13 and the monopole or dipole, and on the effective dielectric constant that reflects how 14 the intervening bonds are polarized.15 Note 2: Although a theoretical distinction may be made between the field effect and 16 the inductive effect as models for the Coulomb interaction between a given site and a 17 remote monopole or dipole within the same entity, the experimental distinction between 18 the two effects has proved difficult, because the field effect and the inductive effect are 19 ordinarily influenced in the same direction by structural changes.20 Note 3: The substituent acts through the electric field that it generates, and it is an 21 oversimplification to reduce that interaction to that of a monopole or dipole.22 See also electronic effect, inductive effect, polar effect.23 See [172,191,199,200].24 rev[3]2526 flash photolysis27 Spectroscopic or kinetic technique in which an ultraviolet, visible, or infrared radiation 28 pulse is used to produce transient species.29 Note 1: Commonly, an intense pulse of short duration is used to produce a sufficient 30 concentration of transient species, suitable for spectroscopic observation. The most 31 common observation is of the absorption of the transient species (transient absorption 32 spectroscopy).33 Note 2: If only photophysical processes are involved, a more appropriate term would 34 be “pulsed photoactivation”. The term “flash photolysis” would be correct only if 35 chemical bonds are broken (the Greek “lysis” means dissolution or decomposition and 36 in general lysis is used to indicate breaking). However, historically, the name has been 37 used to describe the technique of pulsed excitation, independently of the process that 38 follows the excitation.39 See [9].40

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1 flash vacuum pyrolysis (FVP)2 Thermal reaction of a molecule by exposure to a short thermal shock at high 3 temperature, usually in the gas phase.4 Example:

5 See [201,202,203,204].6 rev[3]78 fluxionality9 Property of a chemical species that undergoes rapid degenerate rearrangements

10 (generally detectable by methods that allow the observation of the behaviour of 11 individual nuclei in a rearranged chemical species, e.g., NMR, X-ray).12 Example: tricyclo[3.3.2.02,8]deca-3,6,9-triene (bullvalene), which has 1 209 600 (= 13 10!/3) interconvertible arrangements of the ten CH groups.14

1516 Note 1: Fluxionality differs from resonance, where no rearrangement of nuclear 17 positions occurs.18 Note 2: The term is also used to designate positional change among ligands of 19 complex compounds and organometallics. In these cases the change is not necessarily 20 degenerate.21 See also valence tautomerization.22 rev[3]2324 force-field calculations25 See molecular mechanics calculation.26 GB2728 formal charge29 Quantity (omitted if zero) attached to each atom in a Lewis structure according to3031 Zformal = Nvalence – Nlonepairs – ½ Nbonds

32

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1 Examples: CH2=N+=N–, H3O+, (CH3)2C=N-O–

23 Note: This formalism assumes that electrons in bonds are shared equally, regardless 4 of electronegativity.56 Förster resonance-energy transfer (FRET)7 dipole-dipole excitation transfer8 Nonradiative mechanism for transfer of electronic excitation energy from one molecular 9 entity to another, distant one. It arises from the interaction between the transition dipole

10 moments of the two entities.11 See [9].1213 fractionation factor, isotopic14 Ratio [x1(A)/x2(A)}/[x1(B)/x2(B)], where x is the mole fraction of the isotope designated 15 by the subscript, when the two isotopes are equilibrated between two different chemical 16 species A and B (or between specific sites A and B in the same chemical species).17 Note 1: The term is most commonly met in connection with deuterium solvent isotope 18 effects, where the fractionation factor, symbolized by , expresses the ratio19 = [xD(solute)/xH(solute)}/[xD(solvent)/xH(solvent)]20 for the exchangeable hydrogen atoms in the chemical species (or sites) concerned.21 Note 2: The concept is also applicable to transition states.22 See [2].23 [3]2425 fragmentation26 (1) Heterolytic or homolytic cleavage of a molecule into more than two fragments, 27 according to the general reaction (where a, b, c, d, and X are atoms or groups of atoms)2829 a–b–c–d–X → (a–b)+ + c=d + X–

30 or a–b–c–d–X → (a–b)· + c=d + X31 Examples:32 Ph3C–COOH + H+ → Ph3C+ + C=O + H2O33 CH3N=NCH3 → CH3· + N2 + CH3

34 See [205].3536 (2) Breakdown of a radical or radical ion into a closed-shell molecule or ion and a 37 smaller radical38 Examples:39 (CH3)3C–O· → (CH3)2C=O + H3C·40 [ArBr]·– → Ar· + Br– (solution)

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1 [(CH3)3C–OH]·+ → (CH3)2C=OH+ + H3C· (gas-phase, following ionization)2 [3]34 Franck-Condon Principle5 Approximation that an electronic transition is most likely to occur without change in 6 nuclear positions.7 Note 1: The resulting state is called a Franck–Condon state, and the transition 8 involved is called a vertical transition.9 Note 2: As a consequence, the intensity of a vibronic transition is proportional to the

10 square of the overlap integral between the vibrational wavefunctions of the two states 11 that are involved in the transition.12 See [9].1314 free energy15 The thermodynamic function Gibbs energy (symbol G) or Helmholtz energy (symbol A) 16 specifically defined as 1718 G = G(P,T) = H – TS19 and A = A(V,T) = U – TS2021 where H is enthalpy, U is internal energy, and S is entropy. The possibility of 22 spontaneous motion for a statistical distribution of an assembly of atoms (at absolute 23 temperature T above 0 K) is governed by free energy and not by potential energy.24 Note 1: The IUPAC recommendation is to use the specific terms Gibbs energy or 25 Helmholtz energy whenever possible. However, it is useful to retain the generic term 26 "free energy" for use in contexts where the distinction between (on the one hand) either 27 Gibbs energy or Helmholtz energy and (on the other hand) potential energy is more 28 important than the distinction between conditions either of constant pressure or of 29 constant volume; e.g., in computational modelling to distinguish between results of 30 simulations performed for ensembles under conditions of either constant P or constant 31 V at finite T and calculations based purely on potential energy.32 Note 2: Whereas motion of a single molecular entity is determined by the force acting 33 upon it, which is obtained as the negative gradient of the potential energy, motion for 34 an assembly of many molecular entities is determined by the mean force acting upon 35 the statistical distribution, which is obtained as the negative gradient of the potential of 36 mean force.3738 free radical39 See radical.40 [3]

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12 frontier orbitals3 Highest-energy Occupied Molecular Orbital (HOMO) and Lowest-energy Unoccupied 4 Molecular Orbital (LUMO) of a molecular entity.5 Note 1: These terms should be limited to doubly occupied orbitals, and not to a singly 6 occupied molecular orbital (sometimes designated as a SOMO), because HOMO and 7 LUMO are ambiguous for molecular orbitals that are half filled and thus only partly 8 occupied or unoccupied.9 Note 2: Examination of the mixing of frontier molecular orbitals of reacting molecular

10 entities affords an approach to the interpretation of reaction behaviour; this constitutes 11 a simplified perturbation theory of chemical behaviour.12 Note 3: In some cases a subjacent orbital (Next-to-Highest Occupied Molecular 13 Orbital (NHOMO) or a Second Lowest Unoccupied Molecular Orbital (SLUMO)) may 14 affect reactivity.15 See [8,206,207].16 See also SOMO, subjacent orbital.17 rev[3]1819 frustrated Lewis acid-base pair20 Acid and base for which adduct formation is prevented by steric hindrance.

2122 See [208,209].2324 fullerene25 Molecular entity composed entirely of carbon, in the form of a hollow sphere, ellipsoid, 26 or tube.27 Note: Spherical fullerenes are also called buckyballs, and cylindrical ones are called 28 carbon nanotubes or buckytubes.29 See [40,210].3031 functional group32 Atom or group of atoms within molecular entities that are responsible for the 33 characteristic chemical reactions of those molecular entities. The same functional group 34 will undergo the same or similar chemical reaction(s) regardless of the size of the

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1 molecule it is a part of. However, its relative reactivity can be modified by nearby 2 substituents.3 rev[3]45 gas-phase acidity6 Standard reaction Gibbs energy (∆rGº) change for the gas-phase reaction.

7 A–H → A– + H+

8 Note 1: The symbol often found in the literature is ∆acidG or GA.9 Note 2: The corresponding enthalpy ∆rHo is often symbolized by ∆acidH and called

10 “enthalpy of acidity” or deprotonation enthalpy, abbreviated DPE.11 See [211,212].12 rev[3]1314 gas-phase basicity15 Negative of the standard reaction Gibbs energy (∆rGo) change for the gas-phase 16 reaction

17 B + H+ → BH+

18 Note: An acronym commonly used in the literature is GB. The corresponding 19 enthalpy ∆rHo is called proton affinity, PA, even though affinity properly refers to Gibbs 20 energy. Moreover, such acronyms are not accepted by IUPAC.21 See [213,214].22 rev[3]2324 Gaussian orbital25 Function centred on an atom of the form (r) ∝ xiyjzkexp(–r2), used to approximate 26 atomic orbitals in the LCAO-MO method.27 See [8].2829 geminate pair30 Pair of molecular entities in close proximity within a solvent cage and resulting from 31 reaction (e.g., bond scission, electron transfer, group transfer) of a precursor.32 Note: Because of the proximity the pair constitutes only a single kinetic entity.33 See also ion pair, radical pair.34 rev[3]3536 geminate recombination37 Recombination reaction of a geminate pair.38 Example:

39

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1 rev[3]234 general acid catalysis5 Catalysis of a chemical reaction by Brønsted acids (which may include the solvated 6 hydrogen ion), where the rate of the catalysed part of the reaction is given by HAkHA[HA] 7 multiplied by some function of substrate concentrations.8 Note 1: General acid catalysis can be experimentally distinguished from specific 9 catalysis by hydrogen cations (hydrons) if the rate of reaction increases with buffer

10 concentration at constant pH and ionic strength.11 Note 2: The acid catalysts HA are unchanged by the overall reaction. This 12 requirement is sometimes relaxed, but the phenomenon is then properly called pseudo-13 catalysis.14 See also catalysis, catalytic coefficient, intramolecular catalysis, pseudo-catalysis, 15 specific catalysis.16 rev[3]1718 general base catalysis19 Catalysis of a chemical reaction by Brønsted bases (which may include the lyate ion), 20 where the rate of the catalysed part of the reaction is given by BkB[B] multiplied by 21 some function of substrate concentrations.22 See also general acid catalysis.23 [3]2425 Gibbs energy diagram 26 Diagram showing the relative standard Gibbs energies of reactants, transition states, 27 reaction intermediates, and products, in the same sequence as they occur in a chemical 28 reaction.29 Note 1: The abscissa expresses the sequence of reactants, products, reaction 30 intermediates, and transition states and is often an undefined "reaction coordinate" or 31 only vaguely defined as a measure of progress along a reaction path. In some 32 adaptations the abscissas are however explicitly defined as bond orders, Brønsted 33 exponents, etc.34 Note 2: These points are often connected by a smooth curve (a "Gibbs energy 35 profile", commonly referred to as a "free-energy profile", a terminology that is 36 discouraged), but experimental observation can provide information on relative 37 standard Gibbs energies only at the maxima and minima and not at the configurations 38 between them.39 Note 3: It should be noted that the use of standard Gibbs energies implies a common 40 standard state for all chemical species (usually 1 M for reactions in solution). Contrary

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1 to statements in many textbooks, the highest point on a Gibbs energy diagram does not 2 automatically correspond to the transition state of the rate-limiting step. For example, 3 in a stepwise reaction consisting of two elementary reaction steps4 (1) A + B ⇄ C5 (2) C + D → E6 one of the two transition states must (in general) have a higher standard Gibbs energy 7 than the other. Under experimental conditions where all species have the standard-8 state concentration, then the rate-limiting step is that whose transition state is of highest 9 standard Gibbs energy. However, under (more usual) experimental conditions where

10 all species do not have the standard-state concentration, then the value of the 11 concentration of D determines which reaction step is rate-limiting. However, if the 12 particular concentrations of interest, which may vary, are chosen as the standard state, 13 then the rate-limiting step is indeed the one of highest Gibbs energy.14

151617 See also potential energy profile, potential-energy surface, reaction coordinate.18 rev[3]1920 Gibbs energy of activation (standard free energy of activation)21 ∆‡Gº22 (SI unit: kJ mol–1)23 Standard Gibbs energy difference between the transition state of an elementary 24 reaction and the ground state of the reactants for that step. It is calculated from the rate 25 constant k via the absolute rate equation:26

27 ∆‡Gº = RT [ln( (kB/J K-1)/(h/J s) ) – ln(k/[k]/T/K)]28

29 ∆‡Gº = RT [ln({kB}/{h}) – ln({k}/{T})]30

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1 where kB is Boltzmann's constant, [k] are the units of k, and h Planck's constant. The 2 values of the rate constants, and hence the Gibbs energies of activation, depend upon 3 the choice of concentration units (or of the thermodynamic standard state).4 Note 1: For a complex stepwise reaction, composed of many elementary reactions, 5 ∆‡Gº, the observed Gibbs energy of activation (activation free energy), can be 6 calculated as RT [ln(kB/J K-1)/(h/J s) – ln(k’/[k’]/T/K)], where k’ is the observed rate 7 constant, k’ should be divided by its units, and T/K is the absolute temperature, of 8 dimension 1, since the argument of a logarithmic function should be of dimension 1, as 9 expressed by the reduced variables in the second form of the equation.

10 Note 2: Both ∆‡Gº and k’ are non-trivial functions of the rate constants and activation 11 energies of the elementary steps.12 See also enthalpy of activation, entropy of activation.13 rev[3]1415 graphene16 Allotrope of carbon, whose structure is a one-atom-thick planar sheet of sp2-bonded 17 carbon atoms in a honeycomb (hexagonal) crystal lattice.1819 ground state20 State of lowest Gibbs energy of a system [2].21 Note: In photochemistry and quantum chemistry the lowest-energy state of a 22 chemical entity (ground electronic state) is usually meant.23 See [9].24 See also excited state.25 rev[3]2627 group28 See functional group, substituent.29 rev[3]3031 Grunwald-Winstein equation32 Linear Gibbs-energy relation (Linear free-energy relation)3334 lg(kS/k0) = mY3536 expressing the dependence of the rate of solvolysis of a substrate on the ionizing power 37 of the solvent, where the rate constant k0 applies to the reference solvent (80:20 38 ethanol-water by volume) and kS to the solvent S.

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1 Note 1: The parameter m is characteristic of the substrate and is assigned the value 2 unity for tert-butyl chloride (2-chloro-2-methylpropane). The value Y is intended to be a 3 quantitative measure of the ionizing power of the solvent S.4 Note 2: The equation was later extended [215] to the form56 lg(kS/k0) = mY + lN78 where N is the nucleophilicity of the solvent and l a susceptibility parameter 9 Note 3: The equation has also been applied to reactions other than solvolysis.

10 Note 4: For the definition of other Y-scales, see [216,217,218,219].11 See also Dimroth-Reichardt ET parameter, polarity.12 rev[3]1314 guest15 Organic or inorganic ion or molecule that occupies a cavity, cleft, or pocket within the 16 molecular structure of a host molecular entity and forms a complex with it or that is 17 trapped in a cavity within the crystal structure of a host, but with no covalent bond being 18 formed.19 See also crown ether, cryptand, inclusion compound.20 [3]2122 half-life23 t1/2

24 (SI unit: s)25 Time required for the concentration of a particular reacting chemical species to fall to 26 one-half of its initial value.27 Note 1: Half-life is independent of initial concentration only for a first-order process.28 Note 2: For first-order reactions t1/2 = ln2, where is the lifetime.29 See also lifetime.30 rev[3]3132 halochromism33 Colour change that occurs on addition of acid or base to a solution of a compound as a 34 result of chemical reaction, or on addition of a salt as a result of changing the solvent 35 polarity.36 See [220,221].37 [3]3839 halogen bond

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1 Association between a Lewis-acidic halogen atom in a molecular entity and a Lewis-2 basic region in another, or the same, molecular entity, which acts as an electron-pair 3 donor, such that the balance of forces of attraction and repulsion results in net 4 stabilization.5 Note 1: Typical halogen-bond donors are I2, Br2, ICN, and ICΞCH.6 Note 2: This is analogous to a hydrogen bond, in which the H is the acidic atom.7 Note 3: The interaction provides a stabilization of a few kJ mol–1.8 See [222,223,224,225].9

1011 Hammett equation (Hammett relation)12 Equation of the form1314 lg {kX} = X + lg {k0}15 or lg {KX} = X + lg {K0}1617 expressing the influence of meta and para substituents X on the reactivity of the 18 functional group Y in the benzene derivatives m- and p-XC6H4Y, where {kX} and {KX} 19 are the numerical values of the rate or equilibrium constant, respectively, for the 20 reactions of m- and p-XC6H4Y, X is the substituent constant characteristic of m- or p-21 X, and is the reaction constant characteristic of the given reaction of Y. The values lg 22 {k0} and lg {K0} are intercepts for graphical plots of lg {kX} or lg {KX}, respectively, against 23 X for all substituents X (including X = H, if present). 24 Note 1: It is very common in the literature to find either of these equations written 25 without the curly brackets denoting a reduced rate or equilibrium constant, i.e., k or K 26 divided by its unit, [k] or [K], respectively. 27 Note 2: The Hammett equation is most frequently used to obtain the value of which, 28 as the slope of a graph or least-squares fit, is independent of the choice of units for kX 29 or KX. If the equation is used to estimate the unknown value of a rate or equilibrium 30 constant for an X-substituted compound based upon the known value of X, the ordinate 31 must be multiplied by [kX] or [KX] in order to obtain the value with its appropriate units.32 Note 3: The equation is often encountered in a form with kH or KH incorporated into 33 the logarithm on the left-hand side, where kH or KH corresponds to the reaction of parent 34 C6H5Y, with X = H;3536 lg(kX/kH) = X

37 or lg(KX/KH) = X

3839 This form satisfies the requirement that arguments of the lg function must be of 40 dimension 1, but it would suggest a one-parameter linear least-squares fit, whereas lg

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1 k0, lg K0, lg k0/[k0] and lg K0/[K0], in the other forms correctly represent the intercept in 2 a two-parameter linear least-squares fit of lg k, lg K, lg k/[k], or lg K/[K] vs. X. 3 See [21,22,126,191,226].4 See also extended Hammett equation, Taft equation, Yukawa-Tsuno equation, -5 constant, -value. 6 rev[3]789

1011 Hammond postulate (Hammond-Leffler principle)12 Hypothesis that, when a transition state leading to a high-energy reaction intermediate 13 (or product) has nearly the same energy as that intermediate (or product), the two are 14 interconverted with only a small reorganization of molecular structure.15 Note 1: Essentially the same idea is sometimes referred to as "Leffler's assumption", 16 namely, that the transition state bears a greater resemblance to the less stable species 17 (reactant or reaction intermediate/product). Many textbooks and physical organic 18 chemists, however, express the idea in Leffler's form (couched in terms of Gibbs 19 energies) but attribute it to Hammond (whose original conjecture concerns structure). 20 Note 2: As a corollary, it follows that a factor stabilizing a reaction intermediate will 21 also stabilize the transition state leading to that intermediate.22 Note 3: If a factor stabilizes a reaction intermediate (or reactant or product), then the 23 position of the transition state along the minimum-energy reaction path (MERP) for that 24 elementary step moves away from that intermediate, as shown in the energy diagram 25 (a) below left, where the transition state moves from ‡ to ‡’ when the species to the 26 right is stabilized. This behaviour is often called a Hammond effect and is simply a 27 consequence of adding a linear perturbation to the parabola.28

2930 Note 4: If a structure lying off the MERP is stabilized, then the position of the 31 transition state moves toward that structure, as shown in the energy diagram (b) above

(a) (b)

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1 right, where the transition state moves from ‡ to ‡’. This behaviour is often called anti-2 Hammond and arises because the transition state is a maximum along the MERP but 3 a minimum perpendicular to it.4 See [227,228,229,230].5 See Leffler's relation, More O'Ferrall – Jencks diagram, parallel effect, perpendicular 6 effect.7 rev[3]89 Hansch constant

10 Measure of the contribution of a substituent to the partition ratio of a solute, defined as1112 X = lg(PX/PH)1314 where PX is the partition ratio for the compound with substituent X and PH is the partition 15 constant for the parent.16 See [231,232].17 rev[3]1819 hapticity20 Topological description for the number of contiguous atoms of a ligand that are bonded 21 to a central metal atom.22 Note: The hapticity is indicated as a superscript following the Greek letter .23 Example: (C5H5)2Fe (ferrocene) = bis(5-cyclopentadienyl)iron, where 5 can be read 24 as eta-five or pentahapto.25 See [29].2627 hard acid, base28 Lewis acid with an acceptor centre or Lewis base with a donor centre (e.g., an oxygen 29 atom) of low polarizability.30 Note 1: A high polarizability characterizes a soft acid or base.31 Note 2: Whereas the definition above is qualitative, a theoretically consistent 32 definition of hardness, in eV, is as half the second derivative of the calculated energy 33 E with respect to N, the number of electrons, at constant potential due to the nuclei.34

35 𝜂 = 12 (∂2𝐸

∂𝑁2)𝜈

36

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1 Note 3: Other things being equal, complexes of hard acids with hard bases or of soft 2 acids with soft bases have an added stabilization (sometimes called the HSAB rule). 3 For limitations of the HSAB rule, see [36,233].4 See [234,235,236].5 rev[3]67 HBA solvent8 Strongly or weakly basic solvent capable of acting as a hydrogen-bond acceptor (HBA) 9 and forming strong intermolecular solute-solvent hydrogen bonds.

10 Note: A solvent that is not capable of acting as a hydrogen-bond acceptor is called 11 a non-HBA solvent.12 See [17].1314 HBD solvent (Hydrogen-Bond Donating solvent, also dipolar HBD solvent and protic 15 solvent)16 Solvent with a sizable permanent dipole moment that bears suitably acidic hydrogen 17 atoms to form strong intermolecular solvent-solute hydrogen bonds.18 Note: A solvent that is not capable of acting as hydrogen-bond donor is called a non-19 HBD solvent (formerly aprotic solvent).20 See [17,155].2122 heat capacity of activation23 CP

‡

24 (SI unit: J mol–1 K–1)25 Temperature coefficient of ∆‡H (enthalpy of activation) or ∆‡S (entropy of activation) at 26 constant pressure according to the equations:2728 CP

‡ = (∂∆‡H/∂T)P = T(∂∆‡S/∂T)P

2930 See [237]. 31 rev[3]3233 Henderson-Hasselbalch equation34 Equation of the form3536 pH = pKa – lg([HA]/[A–])3738 relating the pH of a buffer solution to the ratio [HA]/[A–] and the dissociation constant of 39 the acid Ka.40 See [238].

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1 rev[3]23 heterobimetallic complex4 Metal complex having two different metal atoms or ions.5 rev[3]67 heteroleptic8 Characteristic of a transition metal or Main Group compound having more than one type 9 of ligand.

10 See also homoleptic.11 [3]121314 heterolysis, heterolytic bond fission15 Cleavage of a covalent bond so that both bonding electrons remain with one of the two 16 fragments between which the bond is broken, e.g.,

1718 See also heterolytic bond-dissociation energy, homolysis.19 [3]2021 heterolytic bond-dissociation energy22 Energy required to break a given bond of a specific compound by heterolysis.23 Note: For the dissociation of a neutral molecule AB in the gas phase into A+ and B– 24 the heterolytic bond-dissociation energy D(A+B–) is the sum of the homolytic bond-25 dissociation energy, D(A–B), and the adiabatic ionization energy of the radical A· minus 26 the electron affinity of the radical B·.27 [3]2829 high-throughput screening30 Automated method to quickly assay large libraries of chemical species for the affinity of 31 small organic molecules toward a target of interest.32 Note: Currently more than 105 different compounds can be tested per day.33 See [239].3435 highest occupied molecular orbital (HOMO)36 Doubly filled molecular orbital of highest energy.37 Note: Examination of the HOMO can predict whether an electrocyclic reaction is 38 conrotatory or disrotatory.39 See also frontier orbitals.40

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1 Hildebrand solubility parameter [symbol , derived unit: Pa1/2 = (kg m–1 s–2)1/2]2 Ability of a solvent to dissolve a non-electrolyte, defined as the square root of the 3 solvent's cohesive energy density (also called cohesive pressure, equal to the energy 4 of vaporization divided by the solvent’s molar volume and corresponding to the energy 5 necessary to create a cavity in the solvent).6 See [240].7 rev[3]89 Hofmann rule

10 Observation that when two or more alkenes can be produced in a -elimination reaction, 11 the alkene having the smallest number of alkyl groups attached to the double bond 12 carbon atoms is the predominant product.13 Note: This orientation is observed in elimination reactions of quaternary ammonium 14 salts and tertiary sulfonium (sulfanium) salts, and in certain other cases where there is 15 steric hindrance.16 See [241].17 See also Saytzeff rule.18 rev[3]1920 HOMO21 (1) Acronym for Highest Occupied Molecular Orbital.22 See also frontier orbitals.23 (2) Prefix (in lower case) used to indicate a higher homologue of a compound, as 24 homocysteine for HSCH2CH2CH(NH2)COOH, the homologue of cysteine, 25 HSCH2CH(NH2)COOH.26 rev[3]2728 homoaromatic29 Showing features of aromaticity despite a formal discontinuity in the overlap of a cyclic 30 array of p orbitals resulting from the presence of an sp3-hybridized atom at one or 31 several positions within the cycle, in contrast to an aromatic molecule, where there is a 32 continuous overlap of p orbitals over a cyclic array.33 Note 1: Homoaromaticity arises because p-orbital overlap can bridge an sp3 centre.34 Note 2: Pronounced homoaromaticity is not normally associated with neutral 35 molecules, but mainly with ionic species, e.g., the "homotropylium" cation, C8H9

+,

36

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1 Note 3: In bis-, tris-, (etc.) homoaromatic species, two, three, (etc.) single sp3 centres 2 separately interrupt the π-electron system.3 [3]45 homodesmotic reaction6 Subclass of isodesmic reactions in which reactants and products contain not only the 7 same number of carbon atoms in each state of hybridization but also the same number 8 of each CHn group joined to n hydrogen atoms.9 Examples:

1011 cyclo-(CH2)3 + 3 CH3CH3 ⇄ 3 CH3CH2CH3 [3CH2, 6CH3]12 C6H6 (benzene) + 3 CH2=CH2 ⇄ 3 CH2=CH–CH=CH2 [6CH, 6CH2]1314 Note: The definition may be extended to molecules with heteroatoms.15 See [8].16 rev[3]1718 homoleptic19 Characteristic of a transition-metal or Main Group compound having only one type of 20 ligand, e.g., Ta(CH3)5

21 See also heteroleptic.22 [3]2324 homolysis25 Cleavage of a bond so that each of the molecular fragments between which the bond 26 is broken retains one of the bonding electrons.27 Note 1: A unimolecular reaction involving homolysis of a bond not forming part of a 28 cyclic structure in a molecular entity containing an even number of (paired) electrons 29 results in the formation of two radicals:

3031 Note 2: Homolysis is the reverse of colligation.32 See also bond dissociation energy, heterolysis.33 [3]3435 host36 Molecular entity that forms complexes (adducts) with organic or inorganic guests, or a 37 chemical species that can accommodate guests within cavities of its crystal structure.38 Examples include cryptands and crown ethers (where there are ion/dipole attractions 39 between heteroatoms and cations), hydrogen-bonded molecules that form clathrates

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1 (e.g., hydroquinone or water), and host molecules of inclusion compounds (e.g., urea 2 or thiourea), where intermolecular forces and hydrophobic interactions bind the guest 3 to the host molecule.4 rev[3]56 Hückel molecular orbital (HMO) theory7 Simplest molecular orbital theory of -conjugated molecular systems. It uses the 8 following approximations: -electron approximation; LCAO representation of the 9 molecular orbitals; neglect of electron-electron and nuclear-nuclear repulsions. The

10 diagonal elements of the effective Hamiltonian (Coulomb integrals) and the off-diagonal 11 elements for directly bonded atoms (resonance integrals) are taken as empirical 12 parameters, all overlap integrals being neglected.13 See [8].14 rev[3]1516 Hückel (4n + 2) rule17 Principle that monocyclic planar (or almost planar) systems of trigonally (or sometimes 18 digonally) hybridized atoms that contain (4n + 2) electrons (where n is an integer, 19 generally 0 to 5) exhibit aromatic character.20 Note 1: This rule is derived from Hückel MO theory calculations on planar monocyclic 21 conjugated hydrocarbons (CH)m where integer m is at least 3, according to which (4n + 22 2) electrons are contained in a closed-shell system.23 Examples:

2425 Note 2: Planar systems containing 4n electrons (such as cyclobutadiene and 26 cyclopentadienyl cation) are antiaromatic.27 Note 3: Cyclooctatetraene, with 8 electrons, is nonplanar and therefore neither 28 aromatic nor antiaromatic but nonaromatic.29 See [8].30 See also conjugation, Möbius aromaticity.31 rev[3]3233 hybrid orbital34 Atomic orbital constructed as a linear combination of atomic orbitals on an atom.

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1 Note 1: Hybrid orbitals are often used to describe the bonding in molecules 2 containing tetrahedral (sp3), trigonal (sp2), and digonal (sp) atoms, whose bonds are 3 constructed using 1:3, 1:2, or 1:1 combinations, respectively, of s and p atomic orbitals.4 Note 2: Construction of hybrid orbitals can also include d orbitals, as on an octahedral 5 atom, with d2sp3 hybridization.6 Note 3: Integer ratios are not necessary, and the general hybrid orbital made from s 7 and p orbitals can be designated as sp.89 hydration

10 Addition of water or of the elements of water (i.e., H and OH) to a molecular entity or to 11 a chemical species.12 Example: hydration of ethene:13 CH2=CH2 + H2O CH3CH2OH14 Note: In contrast to aquation, hydration, as in the incorporation of waters of 15 crystallization into a protein or in the formation of a layer of water on a nonpolar surface, 16 does not necessarily require bond formation.17 See [48].18 See also aquation.19 rev[3]2021 hydrogen bond22 An attractive interaction between a hydrogen atom from a molecule or a molecular 23 fragment X–H in which X is more electronegative than H, and an atom or a group of 24 atoms in the same or a different molecule, in which there is evidence of bond formation.25 Note 1: Both electronegative atoms are usually (but not necessarily) from the first 26 row of the Periodic Table, i.e., N, O, or F.27 Note 2: A hydrogen bond is largely an electrostatic interaction, heightened by the 28 small size of hydrogen, which permits proximity of the interacting dipoles or charges.29 Note 3: Hydrogen bonds may be intermolecular or intramolecular.30 Note 4: With few exceptions, usually involving hydrogen fluoride and fluoride and 31 other ions, the associated energies are less than 20-25 kJ mol-1.32 Note 5: Hydrogen bonds are important for many chemical structures, giving rise to 33 the attraction between H2O molecules in water and ice, between the strands of DNA, 34 and between aminoacid residues in proteins.35 See [242,243].36 rev[3]3738 hydrolysis39 Solvolysis by water, generally involving the rupture of one or more bonds in the reactant 40 and involvement of water as nucleophile or base.

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1 Example: CH3C(=O)OCH2CH3 + H2O ➛ CH3C(=O)OH + CH3CH2OH2 rev[3]34 hydron5 General name for the ion H+ either in natural abundance or where it is not desired to 6 distinguish between the isotopes, as opposed to proton for 1H+, deuteron for 2H+ and 7 triton for 3H+.8 See [244].9 [3]

1011 hydronation12 Attachment of the ion H+ either in natural abundance or where it is not desired to 13 distinguish between the isotopes.1415 hydrophilicity16 Capacity of a molecular entity or of a substituent to undergo stabilizing interactions with 17 polar solvents, in particular with water and aqueous mixtures, to an extent greater than 18 with a nonpolar solvent.19 rev[3]2021 hydrophobic interaction22 Tendency of lipophilic hydrocarbon-like groups in solutes to form intermolecular 23 aggregates in an aqueous medium.24 Note: The name arises from the attribution of the phenomenon to the apparent 25 repulsion between water and hydrocarbons. However, the phenomenon is more 26 properly attributed to the effect of the hydrocarbon-like groups to avoid disrupting the 27 favorable water-water interactions.28 [3]2930 hyperconjugation31 Delocalization of electrons between bonds and a network.32 Note 1: The concept of hyperconjugation is often applied to carbenium ions and 33 radicals, where the interaction is between bonds and an unfilled or partially filled p or 34 orbital. Resonance illustrating this for the tert-butyl cation is:

35

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1 Note 2: A distinction is made between positive hyperconjugation, as above, and 2 negative hyperconjugation, where the interaction is between a filled or orbital and 3 adjacent antibonding * orbitals, as for example in the fluoroethyl anion (2-fluoroethan-4 1-ide).

56 Note 3: Historically, conjugation involves only bonds, and hyperconjugation is 7 considered unusual in involving bonds.8 See [245,246,247,248].9 See also , , delocalization.

10 rev[3]1112 hypercoordinated13 Feature of a main-group atom with a coordination number greater than four.14 Example: the pentacoordinate carbon in the carbonium ion CH5

+, where three C–H 15 bonds may be regarded as two-electron bonds and the two electrons in the remaining 16 CH2 fragment are delocalized over three atoms. Likewise, both hydrogens in the CH2 17 fragment are hypercoordinated.18 See [2,8].19 See also agostic.20 rev[3]2122 hypervalency23 Ability of an atom in a molecular entity to expand its valence shell beyond the limits of 24 the Lewis octet rule.25 Examples: PF5, SO3, iodine(III) compounds, and pentacoordinate carbocations 26 (carbonium ions).27 Note: Hypervalent compounds are more common for the second- and subsequent-28 row elements in groups 15-18 of the periodic table. A proper description of the 29 hypervalent bonding implies a transfer of electrons from the central (hypervalent) atom 30 to the nonbonding molecular orbitals of the attached ligands, which are usually more 31 electronegative.32 See [8].33 See also valence.34 rev[3]3536 hypsochromic shift

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1 Shift of a spectral band to higher frequency (shorter wavelength) upon substitution or 2 change in medium.3 Note: This is informally referred to as a blue shift and is opposite to a bathochromic 4 shift ("red shift"), but these historical terms are discouraged because they apply only to 5 visible transitions.6 See [9].7 rev[3]89 identity reaction

10 Chemical reaction whose products are chemically identical with the reactants11 Examples:12 (i) bimolecular exchange reaction of CH3I with I–13 (ii) proton transfer between NH4

+ and NH3

14 (iii) electron transfer between manganate(VI) MnO42– and permanganate MnO4

–.15 See also degenerate rearrangement.16 rev[3]1718 imbalance19 Feature that reaction parameters characterizing different bond-forming or bond-20 breaking processes in the same reaction change to different extents as the transition 21 state is approached (along some arbitrarily defined reaction path).22 Note: Imbalance is common in reactions such as elimination, addition, and other 23 complex reactions that involve proton (hydron) transfer.24 Example: the nitroalkane anomaly, where the Brønsted exponent for hydron 25 removal is smaller than the Brønsted for the nitroalkane as acid, because of 26 imbalance between the extent of bond breaking and the extent of resonance 27 delocalization in the transition state.28 See [249].29 See also Brønsted relation, synchronization (principle of nonperfect 30 synchronization), synchronous.31 rev[3]3233 inclusion compound (inclusion complex)34 Complex in which one component (the host) forms a cavity or, in the case of a crystal, 35 a crystal lattice containing spaces in the shape of long tunnels or channels in which 36 molecular entities of a second chemical species (the guest) are located.37 Note: There is no covalent bonding between guest and host, the attraction being 38 generally due to van der Waals forces. If the spaces in the host lattice are enclosed on 39 all sides so that the guest species is "trapped" as in a cage, such compounds are known 40 as clathrates or cage compounds".

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1 See [40].2 [3]34 induction period5 Initial slow phase of a chemical reaction whose rate later accelerates.6 Note: Induction periods are often observed with radical reactions, but they may also 7 occur in other reactions, such as those where a steady-state concentration of the 8 reactants is not established immediately.9 See [13].

10 [3]1112 inductive effect13 Experimentally observable effect (on rates of reaction, etc.) of a substituent through 14 transmission of charge through a chain of atoms by electrostatics.15 Note: Although a theoretical distinction may be made between the field effect and 16 the inductive effect as models for the Coulomb interaction between a given site within 17 a molecular entity and a remote monopole or dipole within the same entity, the 18 experimental distinction between the two effects has proved difficult (except for 19 molecules of peculiar geometry, which may exhibit "reversed field effects"), because 20 the inductive effect and the field effect are ordinarily influenced in the same direction by 21 structural changes.22 See [171,172,250].23 See also field effect, polar effect.24 rev[3]2526 inert27 Stable and unreactive under specified conditions.28 [3]2930 inhibition31 Decrease in rate of reaction brought about by the addition of a substance (inhibitor), by 32 virtue of its effect on the concentration of a reactant, catalyst, or reaction intermediate.33 For example, molecular oxygen or 1,4-benzoquinone can act as an inhibitor in many 34 chain reactions involving radicals as intermediates by virtue of its ability to act as a 35 scavenger toward those radicals.36 Note: If the rate of a reaction in the absence of inhibitor is vo and that in the presence 37 of a certain amount of inhibitor is v, the degree of inhibition (i) is given by3839 i = (vo – v)/vo

40

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1 See also mechanism-based inhibition.2 [3]34 initiation5 Reaction or process generating radicals (or some other reactive reaction intermediates) 6 which then induce a chain reaction or catalytic cycle.7 Example: In the chlorination of alkanes by a radical mechanism the initiation step 8 may be the dissociation of molecular chlorine.9 rev[3]

1011 inner-sphere (electron transfer)12 Feature of an electron transfer between two metal centres that in the transition state 13 share a ligand or atom in their coordination shells.14 Note: The definition has been extended to any situation in which the interaction 15 between the electron-donor and electron-acceptor centres in the transition state is 16 significant (>20 kJ mol–1).17 See [9].18 See also outer-sphere electron transfer.19 [3]2021 insertion22 Chemical reaction or transformation of the general type2324 X–Z + Y X–Y–Z2526 in which the connecting atom or group Y replaces the bond joining the parts X and Z of 27 the reactant XZ.28 Example: carbene insertion reaction2930 R3C–H + H2C: R3C–CH3

3132 Note: The reverse of an insertion is called an extrusion.33 [3]3435 intermediate (reactive intermediate)36 Molecular entity in a stepwise chemical reaction with a lifetime appreciably longer than 37 a molecular vibration (corresponding to a local potential energy minimum of depth 38 greater than RT, and thus distinguished from a transition state) that is formed (directly 39 or indirectly) from the reactants and reacts further to give (directly or indirectly) the 40 products of a chemical reaction.

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1 See also elementary reaction, reaction step, stepwise reaction. 2 [3]34 intermolecular5 (1) Descriptive of any process that involves a transfer (of atoms, groups, electrons, etc.) 6 or interactions between two or more molecular entities.7 (2) Relating to a comparison between different molecular entities.8 See also intramolecular.9 [3]

1011 internal return12 See ion-pair recombination.13 rev[3]1415 intramolecular16 (1) Descriptive of any process that involves a transfer (of atoms, groups, electrons, etc.) 17 or interactions between different parts of the same molecular entity.18 (2) Relating to a comparison between different groups within the same molecular entity.19 See also intermolecular.20 [3]2122 intramolecular catalysis23 Acceleration of a chemical transformation at one site of a molecular entity through the 24 involvement of another functional ("catalytic") group in the same molecular entity, 25 without that group appearing to have undergone change in the reaction product.26 Note 1: The use of the term should be restricted to cases for which analogous 27 intermolecular catalysis by a chemical species bearing that catalytic group is 28 observable.29 Note 2: Intramolecular catalysis can be detected and expressed in quantitative form 30 by a comparison of the reaction rate with that of a comparable model compound in 31 which the catalytic group is absent, or by measurement of the effective molarity of the 32 catalytic group.33 See also effective molarity, neighbouring group participation.34 [3]3536 intrinsic barrier, Δ‡G0

37 Gibbs energy of activation in the limiting case where ΔrGo = 0, i.e., when the effect of 38 thermodynamic driving force is eliminated, as in an identity reaction, X* + AX → X*A + 39 X, where A may be an atom, ion, or group of atoms or ions, and where the equilibrium 40 constant K is equal to 1.

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1 Note: According to the Marcus equation, originally developed for outer-sphere 2 electron transfer reactions, the intrinsic barrier is related to λ, the reorganization energy 3 of the reaction, by the equation45 Δ‡Gº = /467 For a non-identity reaction, Y + AX → YA + X, the intrinsic barrier Δ‡Gº(Y,X) is estimated 8 as ½[Δ‡G(X,X) + Δ‡G(Y,Y)], where the latter two terms are the Gibbs energies of 9 activation of the identity reactions X* + AX → X*A + X and Y* + AY → Y*A + Y,

10 respectively.11 See [251,252,253,254].12 rev[3]1314 intrinsic reaction coordinate (IRC)15 Minimum-energy path leading from the saddle point (corresponding to the transition 16 structure) on the potential-energy surface for an elementary reaction, obtained by 17 tracing the steepest descent in mass-weighted coordinates in both directions.18 Note: The IRC is mathematically well defined, in contrast to the (generally) vague 19 reaction coordinate. Strictly, the IRC is a specific case of a minimum-energy reaction 20 path, and its numerical value at any point along this path is usually taken to be zero at 21 the saddle point, positive in the direction of the products, and negative in the direction 22 of the reactants.23 See [8,255].24 rev[3]2526 inverted micelle (reverse micelle) 27 Association colloid formed reversibly from surfactants in non-polar solvents, leading to 28 aggregates in which the polar groups of the surfactants are concentrated in the interior 29 and the lipophilic groups extend toward and into the non-polar solvent.30 Note: Such association is often of the type31 Monomer ⇄ Dimer ⇄ Trimer ⇄ ··· ⇄ n-mer32 and a critical micelle concentration is consequently not observed.33 [3]3435 ion pair36 Pair of oppositely charged ions held together by coulomb attraction without formation 37 of a covalent bond.38 Note 1: Experimentally, an ion pair behaves as one unit in determining conductivity, 39 kinetic behaviour, osmotic properties, etc.

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1 Note 2: Following Bjerrum, oppositely charged ions with their centres closer together 2 than a distance3

4 d = 𝑍 + 𝑍 ― 𝑒2

4π𝜀0𝜀r𝑘B𝑇56 are considered to constitute an ion pair. Here Z+ and Z– are the charge numbers of the 7 ions, e is the elementary charge, ε0 is the vacuum permittivity, εr is the relative 8 permittivity ("dielectric constant") of the medium, kB is Boltzmann's constant, and T is 9 the absolute temperature. This is the distance at which the Coulomb energy equals the

10 thermal energy, and e2/(40kB) is approximately equal to 1.671 105 m K-1.11 Note 3: An ion pair, the constituent ions of which are in direct contact (and not 12 separated by intervening solvent or by other neutral molecule) is designated as a "tight 13 ion pair" (or "intimate" or "contact ion pair"). A tight ion pair of R+ and X– is symbolically 14 represented as R+X–.15 Note 4: By contrast, an ion pair whose constituent ions are separated by one or 16 several solvent or other neutral molecules is described as a "loose ion pair", 17 symbolically represented as R+||X–. The components of a loose ion pair can readily 18 interchange with other free or loosely paired ions in the solution. This interchange may 19 be detectable (e.g., by isotopic labelling) and thus afford an experimental distinction 20 between tight and loose ion pairs.21 Note 5: A further conceptual distinction has sometimes been made between two 22 types of loose ion pairs. In "solvent-shared ion pairs" for which the ionic constituents of 23 the pair are separated by only a single solvent molecule, whereas in "solvent-separated 24 ion pairs" more than one solvent molecule intervenes. However, the term "solvent-25 separated ion pair" must be used and interpreted with care since it has also widely been 26 used as a less specific term for "loose" ion pair.27 See [256].28 See also common-ion effect, dissociation, ion-pair return, special salt effect.29 [3]3031 ion-pair recombination (formerly ion-pair return)32 Recombination of a pair of ions R+ and X– formed from ionization of RX.33 Note: Ion-pair recombination can be distinguished as external or internal, depending 34 on whether the ion pair did or did not undergo dissociation to free ions.35 See ion pair.36 rev[3]3738 ionic liquid (ionic solvent, molten salt)

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1 Liquid that consists exclusively or almost exclusively of equivalent amounts of 2 oppositely charged ions.3 Note 1: In practice the ions are monocations and monoanions.4 Note 2: Ionic liquids that are liquid at or around room temperature are called room-5 temperature ionic liquids (RTILs).6 Note 3: The term ionic liquid has been often restricted to those water-free liquids that 7 have melting points (or glass-transition temperatures) below 100 °C, following a 8 definition given by Walden [257], who prepared the first ionic liquid, ethylammonium

9 nitrate, CH3CH2NH3+ NO3

−, mp. 13-14 °C, for conductivity measurements.10 Note 4: The terminology for ionic liquids is not yet settled, as stated by Welton [258]. 11 Room-temperature ionic liquid, non-aqueous ionic liquid, molten salt, liquid organic salt, 12 and fused salt are often synonymous.13 See [259,260,261].1415 ionic strength16 I17 (In concentration basis, Ic, SI unit: mol m–3, more commonly mol dm–3 or mol L–1, in 18 molality basis, Im , SI unit: mol kg-1)19 In concentration basis: Ic = 0.5 iciZi

2, in which ci is the concentration of a fully 20 dissociated electrolyte in solution and Zi the charge number of ionic species i.21 In molality basis: Im = 0.5 imiZi

2, in which mi is the molality of a fully dissociated 22 electrolyte in solution and Zi the charge number of ionic species i.23 rev[3]2425 ionization26 Generation of one or more ions.27 Note 1: This may occur, e.g., by loss or gain of an electron from a neutral molecular 28 entity, by the unimolecular heterolysis of that entity into two or more ions, or by a 29 heterolytic substitution reaction involving neutral molecules, such as30 M → M+• + e– (photoionization, electron ionization in mass spectrometry)31 SF6 + e– → SF6

–• (electron capture)32 Ph3CCl → Ph3C+ Cl– (ion pair, which may dissociate to free ions)33 CH3COOH + H2O → H3O+ + CH3CO2

– (dissociation)34 Note 2: In mass spectrometry ions may be generated by several methods, including 35 electron ionization, photoionization, laser desorption, chemical ionization, and 36 electrospray ionization.37 Note 3: Loss of an electron from a singly, doubly, etc. charged cation is called 38 second, third, etc. ionization.39 See also dissociation, ionization energy.40 rev[3]

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12 ionization energy3 Ei

4 (SI unit J)5 Minimum energy required to remove an electron from an isolated molecular entity (in 6 its vibrational ground state) in the gaseous phase.7 Note 1: If the resulting molecular entity is in its vibrational ground state, the energy 8 is the "adiabatic ionization energy".9 Note 2: If the molecular entity produced possesses the vibrational energy determined

10 by the Franck-Condon principle (according to which the electron ejection takes place 11 without an accompanying change in molecular geometry), the energy is the "vertical 12 ionization energy".13 Note 3: The name ionization energy is preferred to the somewhat misleading earlier 14 name "ionization potential".15 See also ionization.16 [3]1718 ionizing power19 Tendency of a particular solvent to promote ionization of a solute.20 Note: The term has been used in both kinetic and thermodynamic contexts.21 See also Dimroth-Reichardt ET parameter, Grunwald-Winstein equation, Z-value.22 [3]2324 ipso attack25 Attachment of an entering group to a position in an aromatic compound already carrying 26 a substituent group other than hydrogen.27 Note: The entering group may displace that substituent group or may itself be 28 expelled or migrate to a different position in a subsequent step. The term "ipso-29 substitution" is not used, since it is synonymous with substitution.30 Example:

3132 where E+ is an electrophile and Z is a substituent other than hydrogen.33 See [262].34 See also cine-substitution, tele-substitution.35 [3]3637 isentropic38 See isoentropic.

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12 isodesmic3 Property of a reaction (actual or hypothetical) in which the types of bonds that are made 4 in forming the products are the same as those that are broken in the reactants.5 Examples:

6 (i) PhCOOH + p-ClC6H4CO2– ⇄ PhCO2

– + p-ClC6H4COOH (1)

7 (ii) 2 CH3F ⇄ CH2F2 + CH4 (2)8 Note 1: Such processes have advantages for theoretical treatment.9 Note 2: The Hammett equation as applied to equilibria, as in (1), succeeds because

10 it deals with isodesmic processes.11 Note 3: For the use of isodesmic processes in quantum chemistry, see [263].12 See [8].13 See also homodesmotic.14 rev[3]1516 isoelectronic17 Having the same number of valence electrons and the same structure, i.e., number and 18 connectivity of atoms, but differing in some or all of the elements involved.19 Examples:20 (i) CO, N2, and NO+

21 (ii) CH2=C=O and CH2=N+=N–

22 [3]2324 isoentropic25 Isentropic 26 Property of a reaction series in which the individual reactions have the same standard 27 entropy or entropy of activation.28 [3]2930 isoequilibrium relationship31 Feature of a series of related substrates or of a single substrate under a series of 32 reaction conditions whereby the enthalpies and entropies of reaction can be correlated 33 by the equation3435 rH – rS = constant3637 Note: The parameter is called the isoequilibrium temperature.38 See [264,265].39 See also compensation effect, isokinetic relationship.

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1 [3]23 isokinetic relationship4 Feature of a series of related substrates or of a single substrate under a series of 5 reaction conditions whereby the enthalpies of activation and entropies of activation can 6 be correlated by the equation7 ‡H – ‡S = constant8 Note 1: The parameter is called the isokinetic temperature. At this temperature all 9 members of the series react at the same rate.

10 Note 2: Isokinetic relationships as established by direct correlation of ‡H with ‡S 11 are often spurious, and the calculated value of is meaningless, because errors in ‡H 12 lead to compensating errors in ‡S. Satisfactory methods of establishing such 13 relationships have been devised.14 See [264,265].15 See also compensation effect, isoequilibrium relationship, isoselective relationship.16 [3]1718 isolobal19 Feature of two molecular fragments for which the number, symmetry properties, 20 approximate energy, shape of the frontier orbitals, and number of electrons in them are 21 similar.22 Example:

23

CH

HMnH LL

L

L

L24 See also isoelectronic.25 rev[3]2627 isomer28 One of several species (or molecular entities) all of which have the same atomic 29 composition (molecular formula) but differ in their connectivity or stereochemistry and 30 hence have different physical and/or chemical properties.31 Note: Conformational isomers that interconvert by rapid rotation about single bonds 32 and configurations that interconvert by rapid pyramidal inversion are often not 33 considered as separate isomers.34 rev[3]3536 isomerization37 Chemical reaction in which the product is an isomer of the reactant.

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1 See also molecular rearrangement.2 rev[3]34 isosbestic point5 Wavelength (or frequency) at which two or more components in a mixture have the 6 same molar absorption coefficients. 7 Note 1: Isosbestic points are commonly met when electronic spectra are taken (a) 8 on a solution in which a chemical reaction is in progress (in which case the two 9 absorbing components concerned are a reactant and a product, A → B), or (b) on a

10 solution in which the two absorbing components are in equilibrium and their relative 11 proportions are controlled by the concentration of some other component, typically the 12 concentration of hydrogen ions, as in an acid-base indicator equilibrium.13 Note 2: The effect may also appear (c) in the spectra of a set of solutions of two or 14 more non-interacting components having the same total concentration.15 Note 3: In all these examples, A (and/or B) may be either a single chemical species 16 or a mixture of chemical species present in invariant proportion.17 Note 4: If absorption spectra of the types considered above intersect not at one or 18 more isosbestic points but over a progressively changing range of wavelengths, this is 19 prima facie evidence (a) for the formation of a reaction intermediate in substantial 20 concentration (A → C → B) or (b) for the involvement of a third absorbing species in 21 the equilibrium, e.g.,22 A ⇄ B + H+

aq ⇄ C + 2 H+aq

23 or (c) for some interaction of A and B, e.g.,24 A + B ⇄ C25 Note 5: Isobestic is a misspelling and is discouraged.26 rev[3]2728 isoselective relationship29 Relationship analogous to the isokinetic relationship, but applied to selectivity data of 30 reactions.31 Note: At the isoselective temperature, the selectivities of the series of reactions 32 following the relationship are identical, within experimental error.33 See [266].34 See also isoequilibrium relationship, isokinetic relationship.35 [3]3637 isotope effect38 Relative difference between, or the ratio of, the rate coefficients or equilibrium constants 39 of two reactions that differ only in the isotopic composition of one or more of their 40 otherwise chemically identical components.

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1 Note: The ratio kl/kh of rate constants (or Kl/Kh of equilibrium constants) for “light” and 2 “heavy” reactions is most often used in studies of chemical reaction mechanisms. 3 However, the opposite ratios are frequently used in environmental chemistry and 4 geochemistry; i.e., kh/kl or Kh/Kl. Furthermore the relative difference kh/kl – 1 or Kh/Kl – 5 1 (often expressed as the percentage deviation of the ratio from unity) is commonly 6 used to quantify isotopic fractionation.7 See kinetic isotope effect, equilibrium isotope effect.8 rev[3]9

10 isotope effect, equilibrium11 isotope effect, thermodynamic12 Effect of isotopic substitution on an equilibrium constant13 Note 1: For example, the effect of isotopic substitution in reactant A that participates 14 in the equilibrium:15 A + B ⇄ C16 is the ratio Kl/Kh of the equilibrium constant for the reaction in which A contains the light 17 isotope to that in which it contains the heavy isotope. The ratio can also be expressed 18 as the equilibrium constant for the isotopic exchange reaction:19 Al + Ch ⇄ Ah + Cl

20 in which reactants such as B that are not isotopically substituted do not appear.21 Note 2: The potential-energy surfaces of isotopic molecules are identical to a high 22 degree of approximation, so thermodynamic isotope effects can arise only from the 23 effect of isotopic mass on the nuclear motions of the reactants and products, and can 24 be expressed quantitatively in terms of nuclear partition functions:2526 Kl/Kh = [Ql(C)/Qh(C)]/ [Ql(A)/Qh(A)]2728 Note 3: Although the nuclear partition function is a product of the translational, 29 rotational, and vibrational partition functions, the isotope effect is usually determined 30 almost entirely by the last named, specifically by vibrational modes involving motion of 31 isotopically different atoms. In the case of light atoms (i.e., protium vs. deuterium or 32 tritium) at moderate temperatures, the isotope effect is dominated by zero-point energy 33 differences.34 See [267].35 See also fractionation factor.36 rev[3]3738 isotope effect, heavy-atom39 Isotope effect due to isotopes other than those of hydrogen.40 [3]

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12 isotope effect, intramolecular3 Kinetic isotope effect observed when a single substrate, in which the isotopic atoms 4 occupy equivalent reactive positions, reacts to produce a non-statistical distribution of 5 isotopologue products.67 Example: PhCH2D + Br· → BrH + PhCHD· vs. BrD + PhCH2·89 The intramolecular isotope effect kH/kD can be measured from the D content of product

10 (PhCH2Br + PhCHDBr), which is experimentally much easier than measuring the 11 intermolecular isotope effect kH/kD from the separate rates of reaction of PhCH3 and 12 PhCD3.13 rev[3]1415 isotope effect, inverse16 Kinetic isotope effect in which kl/kh < 1, i.e., the heavier substrate reacts more rapidly 17 than the lighter one, as opposed to the more usual "normal" isotope effect, in which kl/kh 18 > 1.19 Note: The isotope effect will be "normal" when the vibrational frequency differences 20 between the isotopic transition states are smaller than in the reactants. Conversely, an 21 inverse isotope effect can be taken as evidence for an increase in the force constants 22 on passing from the reactant to the transition state.23 [3]2425 isotope effect, kinetic26 Effect of isotopic substitution on a rate constant.27 For example in the reaction28 A + B → C29 the effect of isotopic substitution in reactant A is expressed as the ratio of rate 30 constants kl/kh, where the superscripts l and h represent reactions in which the 31 molecules A contain the light and heavy isotopes, respectively.32 Note 1: Within the framework of transition-state theory, where the reaction is 33 rewritten as34 A + B ⇄ [TS]‡ → C35 kl/kh can be regarded as if it were the equilibrium constant for an isotope exchange 36 reaction between the transition state [TS]‡ and the isotopically substituted reactant A, 37 and calculated from their vibrational frequencies as in the case of a thermodynamic 38 isotope effect (see thermodynamic (equilibrium) isotope effect).39 Note 2: Isotope effects like the above, involving a direct or indirect comparison of the 40 rates of reaction of isotopologues, are called "intermolecular", in contrast to

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1 intramolecular isotope effects (see intramolecular isotope effect), in which a single 2 substrate reacts to produce a non-statistical distribution of isotopologue product 3 molecules.4 See [267,268].5 [3]67 isotope effect, primary8 Kinetic isotope effect attributable to isotopic substitution of an atom to which a bond is 9 made or broken in the rate-limiting step or in a pre-equilibrium step of a reaction.

10 Note: The corresponding isotope effect on the equilibrium constant of a reaction in 11 which one or more bonds to isotopic atoms are broken is called a primary equilibrium 12 isotope effect.13 See also secondary isotope effect.14 [3]1516 isotope effect, secondary17 Kinetic isotope effect that is attributable to isotopic substitution of an atom to which 18 bonds are neither made nor broken in the rate-limiting step or in a pre-equilibrium step 19 of a specified reaction.20 Note 1: The corresponding isotope effect on the equilibrium constant of such a 21 reaction is called a secondary equilibrium isotope effect.22 Note 2: Secondary isotope effects can be classified as , , etc., where the label 23 denotes the position of isotopic substitution relative to the reaction centre.24 Note 3: Although secondary isotope effects have been discussed in terms of 25 conventional electronic effects, e.g., induction, hyperconjugation, hybridization, such an 26 effect is not electronic but vibrational in origin.27 See [269].28 See also steric isotope effect.29 rev[3]3031 isotope effect, solvent32 Kinetic or equilibrium isotope effect resulting from change in the isotopic composition 33 of the solvent.34 See also kinetic isotope effect, equilibrium isotope effect.35 [3]3637 isotope effect, steric38 Secondary isotope effect attributed to the different vibrational amplitudes of 39 isotopologues.

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1 Note 1: For example, both the mean and mean-square amplitudes of vibrations 2 associated with C–H bonds are greater than those of C–D bonds. The greater effective 3 bulk of molecules containing the former may be manifested by a steric effect on a rate 4 or equilibrium constant. 5 Note 2: Ultimately the steric isotope effect arises from changes in vibrational 6 frequencies and zero-point energies, as normally do other isotope effects.7 rev[3]89 isotope effect, thermodynamic

10 See isotope effect, equilibrium.11 [3]1213 isotope exchange14 Chemical reaction in which the reactant and product chemical species are chemically 15 identical but have different isotopic composition.16 Note: In such a reaction the isotope distribution tends towards equilibrium (as 17 expressed by fractionation factors) as a result of transfers of isotopically different atoms 18 or groups. For example,

19 [3]2021 isotopic perturbation, method of22 Measurement of the NMR shift difference due to the isotope effect on a fast 23 (degenerate) equilibrium between two species that are equivalent except for isotopic 24 substitution.25 Note: This method can distinguish a rapidly equilibrating mixture with time-averaged 26 symmetry from a single structure with higher symmetry.27 See [270,271,272,273].28 [3]2930 isotopic scrambling31 Achievement of, or the process of achieving, a redistribution of isotopes within a 32 specified set of atoms in a chemical species or group of chemical species.

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1 Examples

23 (where * denotes the position of an isotopically different atom.)4 See also fractionation factor.5 [3]67 isotopologues8 Molecular entities that differ only in isotopic composition (number of isotopic 9 substitutions), e.g., CH4, CH3D, CH2D2,

10 Note: These are isotopic homologues. It is a misnomer to call them isotopomers, 11 because they are not isomers with the same atoms.12 rev[3]1314 isotopomers15 Isomers having the same number of each isotopic atom but differing in their positions. 16 The term is a contraction of "isotopic isomer".17 Note: Isotopomers can be either constitutional isomers (e.g., CH2DCH=O and 18 CH3CD=O) or isotopic stereoisomers (e.g., (R)- and (S)-CH3CHDOH or (Z)- and (E)-19 CH3CH=CHD).20 See [11]. 21 [3]2223 Kamlet-Taft solvent parameters24 Quantitative measure of solvent polarity, based on the solvent’s hydrogen-bond donor, 25 hydrogen-bond acceptor, dipolarity/polarizability, and cohesive-pressure properties.26 See [274,275,276,277,278,279].27 See solvent parameter.28 rev[3]2930 Kaptein-Closs rules31 Rules used to predict the sign of chemically induced dynamic nuclear polarization 32 (CIDNP) effects.33 See [280,281,282].

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12 Kekulé structure3 Representation of a molecular entity (usually aromatic) with fixed alternating single and 4 double bonds, in which interactions between multiple bonds are ignored.5 Example: For benzene the Kekulé structures are

67 Note: The distinction among Lewis structure, Kekulé structure, and line formula is 8 now not generally observed, nor is the restriction to aromatic molecular entities.9 See also non-Kekulé structure.

10 rev[3]1112 keto-enol tautomerization13 Interconversion of ketone and enol tautomers by hydron migration, accelerated by acid 14 or base catalysis.15 Example:

1617 See also tautomerization.1819 kinetic ambiguity20 deprecated21 See kinetic equivalence.2223 kinetic control (of product composition)24 Conditions (including reaction times) that lead to reaction products in a proportion 25 governed by the relative rates of the parallel (forward) reactions by which those 26 products are formed, rather than by the equilibrium constants.27 See also thermodynamic control.28 [3]2930 kinetic electrolyte effect31 kinetic ionic-strength effect

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1 General effect of an added electrolyte (i.e., other than, or in addition to, that due to its 2 possible involvement as a reactant or catalyst) on the rate constant of a reaction in 3 solution.4 Note 1: At low concentrations (when only long-range coulombic forces need to be 5 considered) the effect on a given reaction is determined only by the ionic strength of 6 the solution and not by the chemical identity of the ions. This concentration range is 7 roughly the same as the region of validity of the Debye-Hückel limiting law for activity 8 coefficients. At higher concentrations the effect of an added electrolyte depends also 9 on the chemical identity of the ions. Such specific actions can sometimes be interpreted

10 as the incursion of a reaction path involving an ion of the electrolyte as reactant or 11 catalyst, in which case the action is not properly to be regarded just as a kinetic 12 electrolyte effect. At higher concentrations the effect of an added electrolyte does not 13 necessarily involve a new reaction path, but merely the breakdown of the Debye-Hückel 14 law, whereby ionic activity coefficients vary with the ion.15 Note 2: Kinetic electrolyte effects are also called kinetic salt effects.16 Note 3: A kinetic electrolyte effect ascribable solely to the influence of the ionic 17 strength on activity coefficients of ionic reactants and transition states is called a primary 18 kinetic electrolyte effect. A kinetic electrolyte effect arising from the influence of the 19 ionic strength of the solution upon the pre-equilibrium concentration of an ionic species 20 that is involved in a subsequent rate-limiting step of a reaction is called a secondary 21 kinetic electrolyte effect. A common case is the secondary electrolyte effect on the 22 concentration of hydrogen ion (acting as catalyst) produced from the ionization of a 23 weak acid in a buffer solution. To eliminate the complication of kinetic electrolyte effects 24 in buffer solutions, it is advisable to maintain constant ionic strength.25 See [283].26 See also common-ion effect, order of reaction.27 [3]2829 kinetic equivalence30 Property of two reaction schemes that imply the same rate law.31 Example: Schemes (i) and (ii) for the formation of C from A under conditions that B 32 does not accumulate as a reaction intermediate:3334353637383940

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12345 Both equations for d[C]/dt are of the form

67 where r and s are constants (sometimes called rate coefficients). The equations are 8 identical in their dependence on concentrations and do not distinguish whether HO– 9 catalyses the formation of B and its reversion to A, or is involved only in its further

10 transformation to C. The two schemes are therefore kinetically equivalent.11 [3]121314 Koppel-Palm solvent parameters15 Quantitative measure of solvent polarity, based on the solvent’s permittivity, refractive 16 index, basicity or nucleophilicity, and acidity or electrophilicity.17 See [284].18 See solvent parameter.19 rev[3]2021 Kosower Z value22 See Z-value.23 [3]2425 labile26 Property of a chemical species that is relatively unstable and transient or reactive.27 Note: This term must not be used without explanation of the intended meaning.28 See also inert, persistent, reactivity, unreactive.29 [3]3031 Laurence solvent parameters32 Quantitative measures of solvent polarity, based on the solvent’s dispersion/induction 33 interactions, electrostatic interactions between permanent multipoles, solute Lewis 34 base/solvent Lewis acid interactions, and solute HBD/solvent HBA interactions.35 See [285].3637 least nuclear motion, principle of (hypothesis of least motion)

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1 Hypothesis that, for given reactants, the reactions involving the smallest change in 2 nuclear positions will have the lowest energy of activation.3 Note: The basis for this hypothesis is that the energy of a structural deformation 4 leading toward reaction is proportional to the sum of the squares of the changes in 5 nuclear positions, which holds only for small deformations and is therefore not always 6 valid.7 See [8,286].8 rev[3]9

10 leaving group11 Species (charged or uncharged) that carries away the bonding electron pair when it 12 becomes detached from another fragment in the residual part of the substrate.13 Example 1: In the heterolytic solvolysis of (bromomethyl)benzene (benzyl bromide) 14 in acetic acid1516 PhCH2Br + AcOH → PhCH2OAc + HBr1718 the leaving group is Br–.19 Example 2: In the reaction2021 MeS– + PhCH2N+Me3 → MeSCH2Ph + NMe32223 the leaving group is NMe3.24 Note: The historical term “leaving group” is ambiguous, because in the heterolysis of 25 R–X, both R+ and X- are fragments that leave from each other. For that reason, X– (as 26 well as Br– and NMe3 in examples 1 and 2) can unambiguously be called nucleofuges, 27 whereas R+ is an electrofuge.28 See also electrofuge, entering group, nucleofuge.29 rev[3]3031 Leffler's relation32 Leffler’s assumption33 In a series of elementary reactions the changes in Gibbs activation energies are often 34 found to be proportional to the changes in Gibbs energies for the overall reaction.3536 ‡G = rG0

3738 This relation was interpreted in terms of the simple assumption that a small change in 39 any transition-state property P‡ is a linear combination of changes in reactant- and 40 product-state properties, PR and PP.41

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1 P‡ = PP + (1 –) PR

23 Within the limits of this assumption, the parameter is an approximate measure of the 4 fractional displacement of the transition state along the minimum-energy reaction path 5 from reactants to products.6 See [109].7 Note: There are many exceptions to the validity of Leffler’s assumption that is a 8 measure of the position of the transition state.9 See [287].

10 See Hammond postulate.11 rev[3]1213 left-to-right convention14 Arrangement of the structural formulae of the reactants so that the bonds to be made 15 or broken form a linear array in which the electron pushing proceeds from left to right.16 See [288].17 [3]1819 levelling effect20 Tendency of a solvent to make all Brønsted acids whose acidity exceeds a certain value 21 appear equally acidic.22 Note 1: This phenomenon is due to the complete transfer of a hydron to a Brønsted-23 basic solvent from a dissolved acid stronger than the conjugate acid of the solvent. The 24 only acid present to any significant extent in all such solutions is the lyonium ion.25 Note 2: For example, the solvent water has a levelling effect on the acidities of HClO4, 26 HCl, and HI. Aqueous solutions of these acids at the same (moderately low) 27 concentrations have the same acidities.28 Note 3: A corresponding levelling effect applies to strong bases in protogenic 29 solvents.30 [3]3132 Lewis acid33 Molecular entity (and the corresponding chemical species) that is an electron–pair 34 acceptor and therefore able to react with a Lewis base to form a Lewis adduct by 35 sharing the electron pair furnished by the Lewis base.36 Example:3738 Me3B (Lewis acid) + :NH3 (Lewis base) → Me3B––N+H3 (Lewis adduct)3940 See also coordination.

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1 [3]23 Lewis acidity4 Thermodynamic tendency of a substrate to act as a Lewis acid. 5 Note 1: This property is defined quantitatively by the equilibrium constant or Gibbs 6 energy for Lewis adduct formation of a series of Lewis acids with a common reference 7 Lewis base.8 Note 2. An alternative measure of Lewis acidity in the gas phase is the enthalpy of 9 Lewis adduct formation for that Lewis acid with a common reference Lewis base.

10 See also electrophilicity, Lewis basicity.11 rev[3]1213 Lewis adduct14 Adduct formed between a Lewis acid and a Lewis base.15 [3]1617 Lewis base18 Molecular entity (and the corresponding chemical species) able to provide a pair of 19 electrons and thus capable of coordination to a Lewis acid, thereby producing a Lewis 20 adduct.21 [3]2223 Lewis basicity24 Thermodynamic tendency of a substance to act as a Lewis base.25 Note 1: This property is defined quantitatively by the equilibrium constant or Gibbs 26 energy of Lewis adduct formation for that Lewis base with a common reference Lewis 27 acid.28 Note 2: An alternative measure of Lewis basicity in the gas phase is the enthalpy of 29 Lewis adduct formation for that Lewis base with a common reference Lewis acid.30 See also donicity, Lewis acidity, nucleophilicity, proton affinity.31 rev[3]3233 Lewis structure34 electron-dot structure, Lewis formula35 Representation of molecular structure in which (a) nonbonded valence electrons are 36 shown as dots placed adjacent to the atoms with which they are associated and in 37 which (b) a pair of bonding valence electrons in a covalent bond is shown as two dots 38 between the bonded atoms, and in which (c) formal charges (e.g., +, –, 2+) are attached 39 to atoms to indicate the difference between the nuclear charge (atomic number) and

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1 the total number of electrons associated with that atom, on the formal basis that bonding 2 electrons are shared equally between atoms they join.3 Examples:

45 Note 1: A double bond is represented by two pairs of dots, and a triple bond by three 6 pairs, as in the last example above.7 Note 2: Bonding pairs of electrons are usually denoted by lines, representing 8 covalent bonds, as in line formulas, rather than as a pair of dots, and the lone pairs are 9 sometimes omitted.

10 Examples:

1112 Note 3: The distinction among Lewis structure, Kekulé structure, and line formula is 13 now not generally observed.14 rev[3]1516 lifetime (mean lifetime)17 18 Time needed for the concentration of a chemical species that decays in a first-order 19 process to decrease to 1/e of its original value. i.e., c(t = ) = c(t = 0)/e.20 Note 1: Statistically, it represents the mean life expectancy of the species.21 Note 2: Mathematically: = 1/k = 1/(iki) where ki is the first-order rate constant for 22 the i-th decay process of the species.23 Note 3: Lifetime is sometimes applied to processes that are not first-order. However, 24 in such cases the lifetime depends on the initial concentration of the entity or of a 25 quencher and, therefore, only an initial lifetime can be defined. In this case, it should be 26 called decay time.27 See [9].28 See also chemical relaxation, half-life, rate of reaction.29 [3]3031 ligand32 Atom or group bound to a central atom in a polyatomic molecular entity (if it is possible 33 to indicate such a central atom).34 See [29].35 Note 1:The term is generally used in connection with metallic central atoms.36 Note 2: In biochemistry a part of a polyatomic molecular entity may be considered 37 central, and atoms, groups, or molecules bound to that part are considered ligands.

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1 See [289], p. 335.2 [3]34 line formula5 Two-dimensional representation of molecular entities in which atoms are shown joined 6 by lines representing single bonds (or multiple lines for multiple bonds), without any 7 indication or implication concerning the spatial direction of bonds.8 Examples: methanol, ethene

910 See also condensed formula, Kekulé formula, Lewis formula, skeletal formula. 11 rev[3]1213 linear free-energy relation14 linear Gibbs-energy relation15 [3]1617 linear Gibbs-energy relation18 linear free-energy relation (LFER)19 Linear correlation between the logarithm of a rate constant or equilibrium constant for 20 a series of reactions and the logarithm of the rate constant or equilibrium constant for a 21 related series of reactions.22 Typical examples of such relations are the Brønsted relation and the Hammett 23 equation (see also -value).24 Note: The name arises because the logarithm of the value of an equilibrium constant 25 (at constant temperature and pressure) is proportional to a standard Gibbs energy (free 26 energy) change, and the logarithm of the value of a rate constant is a linear function of 27 the Gibbs energy (free energy) of activation.28 [3]2930 linear solvation-energy relationship (LSER)31 Application of solvent parameters in the form of a single- or multi-parameter equation 32 expressing the solvent effect on a given property: e.g., rate of reaction, equilibrium 33 constant, spectroscopic shift.34 Note: The solvent effect may be estimated as a linear combination of elementary 35 effects on a given property P, relative to the property P0 in the reference solvent:3637 P – P0 = a A + b B + p DP…

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12 where A, B, DP, etc. are acidity, basicity, dipolarity, etc. parameters, and a, b, p… are 3 the sensitivity of the property to each effect.4 See Catalán solvent parameters, Dimroth-Reichardt ET parameter, Kamlet-Taft 5 solvent parameters, Koppel-Palm solvent parameters, Laurence solvent parameters, 6 Z-value.7 rev[3]89 line-shape analysis

10 Method for determination of rate constants for dynamic chemical exchange from the 11 shapes of spectroscopic signals, most often used in nuclear magnetic resonance 12 spectroscopy.13 [3]1415 Lineweaver-Burk plot16 See Michaelis-Menten kinetics.17 [3]1819 lipophilic20 Feature of molecular entities (or parts of molecular entities) that have a tendency to 21 dissolve in fat-like solvents (e.g., hydrocarbons).22 See also hydrophilic, hydrophobic interaction.23 rev[3]2425 London forces26 dispersion forces 27 Attractive forces between molecules due to their mutual polarizability.28 Note: London forces are the principal components of the forces between nonpolar 29 molecules.30 See [290].31 See also van der Waals forces.32 [3]3334 lone (electron) pair (nonbonding electron pair)35 Two spin-paired electrons localized in the valence shell on a single atom.36 Note: In structural formulas lone pairs should be designated as two dots.37 See also Lewis structure.38 rev[3]3940 LUMO

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1 Acronym for Lowest Unoccupied Molecular Orbital2 See frontier orbitals.3 rev[3]45 lyate ion6 Anion produced by hydron (proton, deuteron, triton) removal from a solvent molecule.7 Example: the hydroxide ion is the lyate ion of water.8 [3]9

10 lyonium ion11 Cation produced by hydronation (protonation, deuteronation, tritonation) of a solvent 12 molecule.13 Example: CH3OH2

+ is the lyonium ion of methanol.14 See also onium ion.15 [3]16171819 macroscopic diffusion control20 See mixing control.21 [3]2223 magnetic equivalence24 Property of nuclei that have the same resonance frequency in nuclear magnetic 25 resonance spectroscopy and also identical spin-spin interactions with each nucleus of 26 a neighbouring group.27 Note 1: The spin-spin interaction between magnetically equivalent nuclei is not 28 manifested in the spectrum, and has no effect on the multiplicity of the respective NMR 29 signals.30 Note 2: Magnetically equivalent nuclei are necessarily chemically equivalent, but the 31 reverse is not necessarily true.32 [3]3334 magnetization transfer35 NMR method for determining kinetics of chemical exchange by perturbing the 36 magnetization of nuclei in a particular site or sites and following the rate at which 37 magnetic equilibrium is restored.38 Note: The most common perturbations are saturation and inversion, and the 39 corresponding techniques are often called "saturation transfer" and "selective inversion-40 recovery".

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1 See also saturation transfer.2 [3]34 Marcus equation5 General expression that correlates the Gibbs energy of activation (‡G) with the Gibbs 6 energy of the reaction (rGº)78 ‡G = (/4)(1 + rGº/)2 = ‡Go + 1/2 rGº + (rGº)2/(16 ‡Go)9

10 where is the reorganization energy and ‡Go is the intrinsic barrier, with = 4‡Go.11 Note: Originally developed for outer-sphere electron transfer reactions, the Marcus 12 equation applies to many atom- and group-transfer reactions. The Marcus equation 13 captures earlier ideas that reaction thermodynamics can influence reaction barriers: 14 e.g., the Brønsted relation (1926), the Bell-Evans-Polanyi principle (1936-38), Leffler’s 15 relation (1953), and the Hammond postulate (1955) [227]. It also implies that changes 16 in intrinsic barriers may dominate over changes of reaction Gibbs energies and thus 17 account for the fact that reaction rates may not be controlled by the relative 18 thermodynamic stabilities of the products.19 See [291,292,293,294,295,296]. 20 rev[3]2122 Markovnikov (Markownikoff) rule23 Statement of the common mechanistic observation that in electrophilic addition 24 reactions the more electropositive atom (or part) of a polar molecule becomes attached 25 to the carbon bearing more hydrogens.26 Note 1: This rule was originally formulated by Markownikoff (Markovnikov) as "In the 27 addition of hydrogen halides to [unsaturated] hydrocarbons, the halogen atom becomes 28 attached to the carbon bearing the lesser number of hydrogen atoms".29 Note 2: This rule can be rationalized as the addition of the more electropositive atom 30 (or part) of the polar molecule to the end of the multiple bond that would result in the 31 more stable carbenium ion (regardless of whether the carbenium ion is a stable 32 intermediate or a transient structure along the minimum-energy reaction path).33 Note 3: Addition in the opposite sense, as in radical addition reactions, is commonly 34 called anti-Markovnikov addition.35 See [297].36 [3]3738 mass action, law of39 Statement that the velocity of a reaction depends on the active mass, i.e., the 40 concentrations of the reactants.

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1 Example: for an association reaction (1) and its reverse (2)23 A + B AB (1)4 AB A + B (2)56 the forward velocity is v1 = k1 [A][B], with k1 the rate constant for the association reaction. 7 For the dissociation reaction 2 the velocity is v2 = k2 [AB]. This is valid only for 8 elementary reactions. Furthermore, the law of mass action states that, when a 9 reversible chemical reaction reaches equilibrium at a given temperature, the forward

10 rate is the same as the backward rate. Therefore, the concentrations of the chemicals 11 involved bear a constant relation to each other, described by the equilibrium constant, 12 i.e., for 1314 A + B AB 1516 in equilibrium, v1 = k1 [A][B] = v2 = k2 [AB] and one form of the equilibrium constant for 17 the above chemical reaction is the ratio 18

19 Kc = = [AB][A][B]

𝑘1

𝑘2

2021 Note: First recognized in 1864 as the kinetic law of mass action by Guldberg and 22 Waage, who first introduced the concept of dynamic equilibrium, but incorrectly 23 assumed that the rates could be deduced from the stoichiometric equation [298]. Only 24 after the work of Horstmann [299] and van’t Hoff [300] a mathematical derivation of the 25 reaction rates considering the order of the reaction involved was correctly made. 26 See also [301,302,303].27 rev[3]2829 matrix isolation30 Technique for preparation of a reactive or unstable species by dilution in an inert solid 31 matrix (argon, nitrogen, etc.), usually condensed on a window or in an optical cell at low 32 temperature, to preserve its structure for identification by spectroscopic or other means.33 See [304].34 [3]3536 Mayr-Patz equation37 Rate constants for the reactions of sp2-hybridized electrophiles with nucleophiles can 38 be expressed by the correlation39

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1 lg [k/ (dm3 mol-1 s-1)] = sN(E + N),23 where E is the nucleophile-independent electrophilicity parameter, N is the electrophile-4 independent nucleophilicity parameter, and sN is the electrophile-independent 5 nucleophile-specific susceptibility parameter. 6 Note 1: This equation is equivalent to the conventional linear Gibbs energy 7 relationship lg k = Nu + sNE, where Nu = sNN. The use of N is preferred, because it 8 provides an approximate ranking of relative reactivities of nucleophiles.9 Note 2: The correlation should not be applied to reactions with bulky electrophiles,

10 where steric effects cannot be neglected. Because of the way of parametrization, the 11 correlation is applicable only if one or both reaction centres are carbon.12 Note 3: As the E parameters of the reference electrophiles are defined as solvent-13 independent, all solvent effects are shifted into the parameters N and sN.14 Note 4: The equation transforms into the Ritchie equation for sN = 1. 15 Note 5: Applications to SN2-type reactions are possible if an electrophile-specific 16 susceptibility parameter is introduced.17 See [305,306,307,308].18 See also Ritchie equation, Swain-Scott equation.19 The Mayr scale is available at http://www.cup.lmu.de/oc/mayr/DBintro.htm2021 mechanism22 Detailed description of the process leading from the reactants to the products of a 23 reaction, including a characterization as complete as possible of the composition, 24 structure, energy and other properties of reaction intermediates, products, and 25 transition states.26 Note 1: An acceptable mechanism of a specified reaction (and there may be a 27 number of such alternative mechanisms not excluded by the evidence) must be 28 consistent with the reaction stoichiometry, the rate law, and with all other available 29 experimental data, such as the stereochemical course of the reaction. 30 Note 2: Inferences concerning the electronic motions that dynamically interconvert 31 successive species along the reaction path (as represented by curved arrows, for 32 example) are often included in the description of a mechanism.33 rev[3]3435 mechanism-based inhibition (suicide inhibition)36 Irreversible inhibition of an enzyme by formation of covalent bond(s) between the 37 enzyme and the inhibitor, which is a substrate analogue that is converted by the enzyme 38 into a species that reacts with the enzyme.39 rev[3]40

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1 medium2 Phase (and composition of the phase) in which chemical species and their reactions 3 are studied.4 [3]56 Meisenheimer adduct (Jackson-Meisenheimer adduct)7 Lewis adduct formed in nucleophilic aromatic substitution from a nucleophile (Lewis 8 base) and an aromatic or heteroaromatic compound,

910 Note 1: In cases where the substrate lacks electron-withdrawing groups, and 11 depending also on the nucleophile, the Meisenheimer adduct is not a minimum on the 12 potential-energy surface but a transition state.13 See [309,310,311,312]. 14 Note 2: Analogous cationic adducts, such as15

161718 which are intermediates in electrophilic aromatic substitution reactions, are instead 19 called Wheland intermediates or -adducts (or the discouraged term -complexes).20 See [313,314].21 rev[3]2223 melting point (corrected/uncorrected)24 Temperature at which liquid and solid phases coexist in equilibrium, as measured with 25 a thermometer whose reading was corrected (or not) for the emergent stem that is in 26 ambient air.27 Note: In current usage the qualification often means that the thermometer was/(was 28 not) calibrated or that its accuracy was/(was not verified). This usage is inappropriate 29 and should be abandoned.30 [3]3132 mesolytic cleavage

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1 Cleavage of a bond in a radical ion whereby a radical and an ion are formed. The term 2 reflects the mechanistic duality of the process, which can be viewed as homolytic or 3 heterolytic depending on how the electrons are assigned to the fragments.4 See [315,316].5 [3]67 mesomerism, mesomeric structure8 obsolete9 Resonance, resonance form

10 rev[3]1112 mesophase13 Phase of a liquid crystalline compound between the crystalline and the isotropic liquid 14 phase.15 See [317].16 [3]1718 metastable (chemical species)19 deprecated20 Transient (chemical species).21 [3]2223 metathesis24 Process formally involving the redistribution of fragments between similar chemical 25 species so that the bonds to those fragments in the products are identical (or closely 26 similar) to those in the reactants.27 Example:

2829 Note: The term has its origin in inorganic chemistry, with a different meaning, but that 30 older usage is not applicable in physical organic chemistry.31 [3]3233 micellar catalysis34 Acceleration of a chemical reaction in solution by the addition of a surfactant at a 35 concentration higher than its critical micelle concentration so that the reaction can 36 proceed in the environment of surfactant aggregates (micelles).

RCH CHR

RCH CHR

RCH

RCH

RCH

RCH

catalyst

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1 Note 1: Rate enhancements may be due to a higher concentration of the reactants 2 in that environment, or to a more favourable orientation and solvation of the species, or 3 to enhanced rate constants in the micellar pseudophase of the surfactant aggregate.4 Note 2: Micelle formation can also lead to a decreased reaction rate.5 See [318].6 See also catalyst.7 [3]89 micelle

10 Aggregate of 1- to 1000-nm diameter formed by surfactants in solution, which exists in 11 equilibrium with the molecules or ions from which it is formed.12 See [135].13 See also inverted micelle.14 rev[3]1516 Michaelis-Menten kinetics17 Appearance of saturation behavior in the dependence of the initial rate of reaction v0 18 on the initial concentration [S]0 of a substrate when it is present in large excess over 19 the concentration of an enzyme or other catalyst (or reagent) E, following the equation,2021 v0 = Vmax [S]0 /(KM + [S]0),2223 where v0 is the observed initial rate, Vmax is its limiting value at substrate saturation (i.e., 24 when [S]0 >> KM, so that all enzyme is bound to substrate), and KM is the substrate 25 concentration at which v0 = Vmax/2.26 Note 1: Empirical definition, applying to any reaction that follows an equation of this 27 general form.28 Note 2: Often only initial rates are measured, at low conversion, so that [S]0 can be 29 considered as time-independent but varied from run to run.30 Note 3: The parameters Vmax and KM (the Michaelis constant) can be evaluated from 31 slope and intercept of a linear plot of 1/v0 against 1/[S]0 (Lineweaver-Burk plot) or from 32 slope and intercept of a linear plot of [S]0/v0 against [S]0 (Eadie-Hofstee plot), but a 33 nonlinear fit, which is readily performed with modern software, is preferable.34 Note 4: This equation is also applicable to the condition where E is present in large 35 excess, in which case [E] appears in the equation instead of [S]0.36 Note 5: The term has been used to describe reactions that proceed according to the 37 scheme

38

E + S ES Products + Ek1

k-1

kcat

39

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1 in which case KM = (k–1 + kcat)/k1 (Briggs-Haldane conditions). It has more usually been 2 applied only to the special case in which k–1 >> kcat and KM = k–1/k1; in this case KM is a 3 true dissociation constant (Michaelis-Menten conditions).4 See [319,320].5 [3]67 microscopic diffusion control (encounter control)8 Observable consequence of the limitation that the rate of a bimolecular chemical 9 reaction in a homogeneous medium cannot exceed the rate of encounter of the reacting

10 molecular entities.11 Note: The maximum rate constant is usually in the range 109 to 1010 dm3 mol–1 s–1 in 12 common solvents at room temperature.13 See also mixing control.14 rev[3]1516 microscopic reversibility, principle of17 In a reversible reaction the mechanism in one direction is exactly the reverse of the 18 mechanism in the other direction.19 See also chemical reaction, detailed balancing.20 [3]2122 migration23 Transfer (usually intramolecular) of an atom or group during the course of a molecular 24 rearrangement.25 Example (of a hydrogen migration):2627 RCH2CH=CH2 → RCH=CHCH3

28 rev[3]2930 migratory aptitude31 Tendency of a group to participate in a molecular rearrangement, relative to that of 32 another group, often in the same molecule.33 Example: In the Baeyer-Villiger rearrangement of PhCOCH3, via intermediate 34 PhC(OH)(OOCOAr)CH3, the major product is CH3COOPh, by phenyl migration, rather 35 than PhCOOCH3.36 Note: In nucleophilic rearrangements (migration to an electron-deficient centre) the 37 migratory aptitude of a group is loosely related to its capacity to stabilize a partial 38 positive charge, but exceptions are known, and the position of hydrogen in the series is 39 often unpredictable.40 rev[3]

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12 migratory insertion3 Reaction that involves the migration of a group to another position on a metal centre, 4 with insertion of that group into the bond between the metal and the group that is in that 5 other position. 6 Examples:7 CO insertion8

91011 Ziegler-Natta reaction12

13LnM

RLnM

RLnM

CH2CH2RCH2 CH2 migratory

insertion

1415 Note: On repetition of the reaction the substituent alternates between the two 16 positions.17 rev[3]1819 minimum structural change, principle of20 Principle that a chemical reaction is expected to occur with a minimum of bond changes 21 or with a minimum redistribution of electrons (although more complex reaction 22 cascades are also possible).23 Example:24

25

Cl Cl

2627 where the transfer of Cl is accompanied by migration of only one carbon, rather than a 28 more extensive molecular rearrangement involving migration of three methyls.29 See [286]. 30 See also least nuclear motion, principle of.31 rev[3]32

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1 minimum-energy reaction path (MERP)2 Path of steepest-descent from a saddle point on a potential-energy surface in each 3 direction towards adjacent energy minima; equivalent to the energetically easiest route 4 from reactants to products.5 See intrinsic reaction coordinate.6 See [8].78 mixing control9 Experimental limitation of the rate of reaction in solution by the rate of mixing of

10 solutions of the two reactants.11 Note 1: Mixing control can occur even when the reaction rate constant is several 12 orders of magnitude less than that for an encounter-controlled reaction.13 Note 2: Analogous (and more important) effects of the limitation of reaction rates by 14 the rate of mixing are encountered in heterogeneous (solid/liquid, solid/gas, liquid/gas) 15 systems.16 See [321].17 See also microscopic diffusion control, stopped flow.18 [3]19202122 Möbius aromaticity23 Feature of a monocyclic array of orbitals in which there is a single out-of-phase 24 overlap (or, more generally, an odd number of out-of-phase overlaps), whereby the 25 pattern of aromatic character is opposite to Hückel systems; with 4n electrons it is 26 stabilized (aromatic), whereas with 4n + 2 it is destabilized (antiaromatic).27 Note 1: The name is derived from the topological analogy of such an arrangement of 28 orbitals to a Möbius strip.29 Note 2: The concept has been applied to transition states of pericyclic reactions.Note 30 3: In the electronically excited state 4n + 2 Möbius -electron systems are stabilized, 31 and 4n systems are destabilized.32 Note 4: A few examples of ground-state Möbius systems are known [322,323].33 See [324,325]. 34 See also aromatic, Hückel (4n + 2) rule.35 rev[3]3637 molecular entity38 Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical 39 ion, complex, conformer, etc., identifiable as a separately distinguishable entity.

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1 Note 1: Molecular entity is used in this glossary as a general term for singular entities, 2 irrespective of their nature, while chemical species stands for sets or ensembles of 3 molecular entities. Note that the name of a substance may refer to the molecular entity 4 or to the chemical species, e.g., methane, may mean either a single molecule of CH4 5 (molecular entity) or an ensemble of such species, specified or not (chemical species), 6 participating in a reaction.7 Note 2: The degree of precision necessary to describe a molecular entity depends 8 on the context. For example "hydrogen molecule" is an adequate definition of a certain 9 molecular entity for some purposes, whereas for others it may be necessary to

10 distinguish the electronic state and/or vibrational state and/or nuclear spin, etc. of the 11 molecule.12 [3]1314 molecular formula15 List of the elements in a chemical species or molecular entity, with subscripts indicating 16 how many atoms of each element are included.17 Note: In organic chemistry C and H are listed first, then the other elements in 18 alphabetical order.19 See [29].20 See also empirical formula.212223 molecular mechanics (MM) (empirical force-field calculation)24 Computational method intended to give estimates of structures and energies for 25 molecules. 26 Note: Even though such calculations can be made with either classical or quantum 27 mechanics (or both), the term molecular mechanics is widely understood as a classical 28 mechanics method that does not explicitly describe the electronic structure of the 29 molecular entities. It is based on the assumption of preferred bond lengths and angles, 30 deviations from which lead to strain, and the existence of torsional interactions and 31 attractive and repulsive van der Waals and Coulombic forces between non-bonded 32 atoms, all of which are parametrized to fit experimental properties such as energies or 33 structures. In contrast, in the quantum mechanical implementation no such 34 assumptions/parameters are needed.35 See [8,326].36 rev[3]3738 molecular metal39 Non-metallic material whose properties, such as conductivity, resemble those of metals, 40 usually following oxidative doping.

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1 Example: polyacetylene following oxidative doping with iodine.2 [3]34 molecular orbital5 One-electron wavefunction describing a single electron moving in the field provided by 6 the nuclei and all other electrons of a molecular entity of more than one atom.7 Note 1: Molecular orbitals describing valence electrons are often delocalized over 8 several atoms of a molecule. They are conveniently expressed as linear combinations 9 of atomic orbitals. They can be described as two-centre, multi-centre, etc. in terms of

10 the number of nuclei (or "centres") encompassed. 11 Note 2: For molecules with a plane of symmetry, a molecular orbital can be classed 12 as sigma () or pi (), depending on whether the orbital is symmetric or antisymmetric 13 with respect to reflection in that plane.14 See atomic orbital, orbital.15 See [8].16 rev[3]1718 molecular rearrangement19 Reaction of a molecular entity that involves a change of connectivity.20 Note 1: The simplest type of rearrangement is an intramolecular reaction in which 21 the product is isomeric with the reactant (intramolecular isomerization). An example is 22 the first step of the Claisen rearrangement.23

2425 Note 2: The definition of molecular rearrangements includes reactions in which there 26 is a migration of an atom or bond (unexpected on the basis of the principle of minimum 27 structural change), as in the reaction2829 CH3CH2CH2Br + AgOAc → (CH3)2CHOAc + AgBr3031 where the rearrangement step can formally be represented as the "1,2-shift" of hydride 32 between adjacent carbon atoms in a carbocation3334 CH3CH2CH2

+ → (CH3)2CH+

3536 Note 3: Such migrations also occur in radicals, e.g.,

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12 PhC(CH3)2–CH2· → ·C(CH3)2CH2Ph (neophyl rearrangement)34 Note 4: The definition also includes reactions in which an entering group takes up a 5 different position from the leaving group, with accompanying bond migration, such as 6 in the "allylic rearrangement":78 (CH3)2C=CHCH2Br + HO– → (CH3)2C(OH)CH=CH2 + Br–

910 Note 5: A distinction can be made between intramolecular rearrangements (or "true" 11 molecular rearrangements) and intermolecular rearrangements (or apparent 12 rearrangements). In the former case the atoms and groups that are common to a 13 reactant and a product never separate into independent fragments during the 14 rearrangement stage, whereas in an intermolecular rearrangement a migrating group 15 becomes completely free from the parent molecule and is re-attached to a different 16 position in a subsequent step, as in the Orton reaction:1718 C6H5N(Cl)COCH3 + HCl → C6H5NHCOCH3 + Cl2 19 → o- and p-ClC6H4NHCOCH3 + HCl20 See [327].21 rev[3]2223 molecular recognition24 Attraction between specific molecules through noncovalent interactions that often 25 exhibit electrostatic and stereochemical complementarity between the partners26 Note: The partners are usually designated as host and guest, where the host 27 recognizes and binds the guest with high selectivity over other molecules of similar size 28 and shape. 2930 molecularity31 Number of reactant molecular entities that are involved in the "microscopic chemical 32 event" constituting an elementary reaction.33 Note 1: For reactions in solution this number is always taken to exclude molecular 34 entities that form part of the medium and which are involved solely by virtue of their 35 solvation of solutes.36 Note 2: A reaction with a molecularity of one is called "unimolecular", one with a 37 molecularity of two "bimolecular", and of three "termolecular".38 See also chemical reaction, order of reaction.39 [3]40

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1 molecule2 An electrically neutral entity consisting of more than one atom.3 Note: Rigorously, a molecule must correspond to a depression on the potential-4 energy surface that is deep enough to confine at least one vibrational state.5 See also molecular entity.6 [3]78 More O'Ferrall - Jencks diagram9 Conceptual visualization of the potential-energy surface for a reacting system, as a

10 function of two coordinates, usually bond lengths or bond orders.11 Note 1: The diagram is useful for analyzing structural effects on transition-state 12 geometry and energy.13 Note 2: According to the Hammond postulate, stabilization of the products relative to 14 the reactants (an effect parallel to the minimum-energy reaction path, MERP) shifts the 15 transition state away from the product geometry, whereas destabilization of the 16 products shifts the transition state towards the product geometry. As first noted by 17 Thornton, stabilization of a structure located off the assumed MERP in a direction 18 perpendicular to it shifts the transition state toward that more stabilized geometry (a 19 perpendicular effect); destabilization shifts the transition state in the opposite direction.20

21 2223 Figure. More O'Ferrall-Jencks diagram for β-elimination reaction, with reactants, 24 products, and possible intermediates at the four corners.

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12 Case 1: In a concerted elimination the transition state (‡1) is a saddle point near the 3 centre of the diagram, and the assumed MERP follows close to the diagonal.4 Case 2: If the carbanion intermediate is stabilized, the transition state shifts toward 5 that intermediate, to a transition state (‡2) in which the C–H bond is more extensively 6 broken and the C–X bond is more intact. Conversely, if the carbocation intermediate is 7 stabilized, the transition state shifts towards the top-left corner of the Figure, with less 8 C–H bond breaking and more C–X bond making9 Case 3: If leaving group X– is stabilised, energy decreases along the dotted vertical

10 arrow. It follows that in the resulting transition state (‡3) the C–H bond is more intact but 11 there is little change in the C–X bond, the shift of the transition state is the resultant of 12 a parallel (Hammond) component away from the top-right corner and a perpendicular 13 (anti-Hammond) component towards the top-left corner, yielding a transition state (‡3) 14 with less C–H bond breaking but approximately no change in the extent of C–X bond 15 making. 16 See [229,328,329,330,331].17 See also Hammond Postulate, anti-Hammond effect.18 rev[3]1920 Morse potential21 V(r)22 (unit J)23 Empirical function relating the potential energy of a molecule to the interatomic distance 24 r accounting for the anharmonicity of bond stretching:2526 V(r) = De{1 – exp[–a(r–re)]}2

2728 where De is the bond-dissociation energy, re is the equilibrium bond length, and a is a 29 parameter characteristic of a given molecule.30 See [8].3132 mu33 µ34 (1) Symbol used to designate (as a prefix) a ligand that bridges two or more atoms.35 Note: If there are more than two atoms being bridged, μ carries a subscript to denote 36 the number of atoms bridged.37 (2) Symbol used to designate dipole moment as well as many other terms in physics 38 and physical chemistry.3940 multi-centre bond

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1 Bond in which an electron pair is shared among three or more atomic centres.2 Note 1: This may be needed when the representation of a molecular entity solely by 3 localized two-electron two-centre bonds is unsatisfactory, or when there are not enough 4 electrons to allow one electron pair shared between two adjacent atoms.5 Note 2: This is restricted to σ bonds and does not apply to species with delocalized 6 electrons.7 Examples include the three-centre bonds in diborane B2H6 and in bridged 8 carbocations.9 rev[3]

1011 multident12 multidentate13 See ambident.14 [3]1516 nanomaterial17 Substance whose particles are in the size range of 1 to 100 nm.18 Note: This may have chemical properties different from those of the corresponding 19 bulk material.2021 narcissistic reaction22 Chemical reaction that can be described as the automerization or enantiomerization of 23 a reactant into its mirror image (regardless of whether the reactant is chiral). 24 Examples are cited under degenerate rearrangement and fluxional.25 See [332].26 rev[3]2728 neighbouring-group participation29 Direct interaction of the reaction centre (usually, but not necessarily, an incipient 30 carbenium-ion centre) with a lone pair of electrons of an atom or with the electrons of a 31 or bond contained within the parent molecule but not conjugated with the reaction 32 centre.33 Example:

34

RS

X

RS

NuS

RS X Nu

3536 Note 1: A distinction is sometimes made between n, , and participation.37 Note 2: The neighbouring group serves as a nucleophile, as in SN2 reactions, except 38 that the nucleophile is intramolecular, so that this step is unimolecular.

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1 Note 3: A rate increase due to neighbouring-group participation is known as 2 anchimeric assistance.3 Note 4: Synartetic acceleration is the name given to the special case of participation 4 by electrons on a substituent attached to a -carbon, relative to the leaving group 5 attached to the -carbon, as in the example above. This term is deprecated.6 See also intramolecular catalysis, multi-centre bond.7 rev[3]89 NHOMO

10 Next-to-highest occupied molecular orbital.11 See subjacent orbital.12 rev[3]1314 NIH shift15 Intramolecular hydrogen migration that can be observed in enzymatic and chemical 16 hydroxylations of aromatic rings, as evidenced by appropriate deuterium labelling, as 17 in18

19

D

CH3

H

H

H

H

OH

CH3

D

H

H

H

hydroxylase

2021 Note 1: In enzymatic reactions the NIH shift is thought to derive from the 22 rearrangement of arene oxide intermediates, but other pathways have been suggested.23 Note 2: NIH stands for National Institutes of Health, where the shift was discovered.24 See [333].25 [3]2627 nitrene28 Generic name for the species HN and substitution derivatives thereof, containing an 29 electrically neutral univalent nitrogen atom with four nonbonding electrons.30 Note 1: Two nonbonding electrons may have antiparallel spins (singlet state) or 31 parallel spins (triplet state).32 Note 2: The name is the strict analogue to carbene and, as a generic name, it is 33 preferred to a number of alternative proposed (imene, imine radical, aminylene, azene, 34 azylene, azacarbene, imin, imidogen).35 See [97,334].36 [3]37

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1 no-bond resonance2 double-bond-no-bond resonance3 Inclusion of one or more contributing structures that lack the bond of another 4 contributing structure.5 Example:

67 See hyperconjugation.8 rev[3]9

10 nonclassical carbocation11 Carbocation that has delocalized (bridged) bonding electrons.12 See [89,335].13 rev[3]1415 non-Kekulé structure16 Compound with unpaired electrons for which no Lewis structures are possible with all 17 bonding electrons paired in single or double bonds.18 Examples:19

20 H2C CH2

2122 Note: For the second example ("trimethylenemethane") the isomer shown (valence 23 tautomer methylidenecyclopropane) is a Kekulé structure.24 See [336].2526 nucleofuge27 Leaving group that carries away the bonding electron pair in a nucleophilic substitution 28 reaction.29 Example: In the hydrolysis of a chloroalkane, Cl– is the nucleofuge.3031 R–Cl + H2O → ROH + HCl32

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1 Note 1: Nucleofugality, commonly called leaving-group ability, characterizes the 2 relative rates of atoms or groups to depart with the bonding electron pair from a 3 reference substrate. Nucleofugality depends on the nature of the reference reaction and 4 is not the reverse of nucleophilicity.5 Note 2: Protofugality is a special type of nucleofugality, characterizing the relative 6 rates of proton transfer from a series of Brønsted acids H–X to a common Brønsted 7 base. 8 See [194].9 See also electrofuge, nucleophile.

10 rev[3]1112 nucleophile (n.), nucleophilic (adj.)13 Reactant that forms a bond to its reaction partner (the electrophile) by donating both of 14 its bonding electrons.15 Note 1: A "nucleophilic substitution reaction" is a heterolytic reaction in which the 16 reagent supplying the entering group acts as a nucleophile.17 Example:1819 MeO– (nucleophile) + EtCl → MeOEt + Cl– (nucleofuge)2021 Note 2: Nucleophilic reagents are Lewis bases.22 Note 3: The term "nucleophilic" is also used to designate the apparent polar character 23 of certain radicals, as inferred from their higher relative reactivity with reaction sites of 24 lower electron density.25 See also nucleophilicity, order of reaction.26 [3]2728 nucleophilic catalysis29 Catalysis by a Lewis base, involving conversion of a substrate with low electrophilicity 30 into an intermediate with higher electrophilicity. 31 Example 1: hydrolysis of acetic anhydride in aqueous solution, catalysed by 4-32 (dimethylamino)pyridine33 Me2NC5H4N + (CH3CO)2O → (Me2NC5H4NCOCH3)+ + CH3CO2

–

34 then (Me2NC5H4COCH3)+ + H2O → Me2NC5H4N + CH3CO2H + H+aq

3536 Example 2: SN2 reaction of CH3CH2Cl with HO–, catalyzed by I–:3738 CH3CH2Cl + I– → Cl– + CH3CH2I, then CH3CH2I + HO– → CH3CH2OH + I–3940 See also nucleophilicity.

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1 rev[3]23 nucleophilic substitution4 Heterolytic reaction in which an entering group adds to the electrophilic part of the 5 substrate and in which the leaving group, or nucleofuge, retains both electrons of the 6 bond that is broken, whereupon it becomes another potential nucleophile.7 Example:89 CH3Br (substrate) + HO– (nucleophile) → CH3OH + Br– (nucleofuge)

1011 Note 1: It is arbitrary to emphasize the nucleophile and ignore the feature that this is 12 also an electrophilic substitution, but the distinction depends on the nucleophilic nature 13 of the reactant that is considered to react with the substrate.14 Note 2: Nucleophilic substitution reactions are designated as SN1 or SN2, depending 15 on whether they are unimolecular or bimolecular, respectively. Mechanistically, these 16 correspond to two-step and one-step processes, respectively. SN1 reactions follow first-17 order kinetics but SN2 reactions do not always follow second-order kinetics.18 See order of reaction, rate coefficient.192021 nucleophilicity22 Relative reactivity of a nucleophile toward a common electrophile.23 Note 1: The concept is related to Lewis basicity. However, whereas the Lewis 24 basicity of a base B: is measured by its equilibrium constant for adduct formation with 25 a common acid A, the nucleophilicity of a Lewis base B: is measured by the rate 26 coefficient for reaction with a common substrate A–Z, often involving formation of a 27 bond to carbon.28 Note 2: Protophilicity is a special case of nucleophilicity, describing the relative rates 29 of reactions of a series of Lewis bases B: with a common Brønsted acid H–Z. The term 30 “protophilicity” is preferred over the alternative term “kinetic basicity” because basicity 31 refers to equilibrium constants, whereas “philicity” (like “fugality”) refers to rate 32 constants.33 See [192,193,307].34 See also Brønsted basicity, electrophilicity, Lewis basicity, Mayr-Patz equation, 35 Ritchie equation, Swain-Scott equation.36 rev[3]3738 n-* delocalization (n-* no-bond resonance)39 Delocalization of a lone pair (n) into an antibonding -orbital (*).40 See also anomeric effect, hyperconjugation, resonance.

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1 [3]23 octanol-water partition ratio (KOW):4 Equilibrium concentration of a substance in octan-1-ol divided by its equilibrium 5 concentration in water. 6 Note: This is a measure of the lipophilicity of a substance. It is used in 7 pharmacological studies and in the assessment of environmental fate and transport of 8 organic chemicals.9 See [337,338].

10 See also partition ratio.1112 octet rule13 Electron-counting rule that the number of lone-pair electrons on a first-row atom plus 14 the number of electron pairs in that atom's bonds should be 8.1516 onium ion17 (1) Cation derived by addition of a hydron to a mononuclear parent hydride of the 18 nitrogen, chalcogen, and halogen family, e.g., H4N+ ammonium ion.19 (2) Derivative formed by substitution of the above parent ions by univalent groups, e.g., 20 (CH3)2SH+ dimethylsulfonium (dimethylsulfanium), (CH3CH2)4N+ tetraethylammonium.21 See [97].22 See also carbenium ion, carbonium ion.23 rev[3]2425 optical yield26 Ratio of the optical purity of the product to that of the chiral precursor or reactant.27 Note 1: This should not be confused with enantiomeric excess. 28 Note 2: The optical yield is not related to the chemical yield of the reaction. 29 See [11]. 30 See stereoselectivity.31 [3]3233 orbital (atomic or molecular)34 Wavefunction depending on the spatial coordinates of only one electron.35 Note: An orbital is often illustrated by sketching contours, often very approximate, on 36 which the wavefunction has a constant value or by indicating schematically the 37 envelope of the region of space in which there is an arbitrarily fixed high probability (say 38 95 %) of finding the electron occupying that region, and affixing also the algebraic sign 39 (+ or –) of the wavefunction in each part of that region, or suggesting the sign by 40 shading.

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1 Examples:2

345 See atomic orbital, molecular orbital.6 See [8].7 rev[3]89 orbital steering

10 Concept expressing the principle that the energetically favourable stereochemistry of 11 approach of two reacting species is governed by the most favourable overlap of their 12 appropriate orbitals.13 rev[3]1415 orbital symmetry 16 Behaviour of an atomic orbital or molecular orbital under molecular symmetry 17 operations, such that under reflection in a symmetry plane or rotation by 180º around a 18 symmetry axis the phase of the orbital is either unchanged (symmetric) or changes sign 19 (antisymmetric), whereby positive and negative lobes are interchanged.20 Examples:21 (1) The orbital of an idealized single bond is , with cylindrical symmetry.22 (2) A p-orbital or -bond orbital has symmetry, i.e., it is antisymmetric with respect 23 to reflection in a plane passing through the atomic centres with which it is associated. 24 (3) The HOMO of 1,3-butadiene (illustrated below by its component atomic orbitals) 25 is antisymmetric with respect to 180º rotation about an axis through the C2–C3 bond 26 and perpendicular to the molecular plane.27282930313233 See [107,123].34 See also conservation of orbital symmetry, sigma, pi.35 rev[3]3637 order of reaction

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1 Exponent , independent of concentration and time, in the differential rate equation 2 (rate law) relating the macroscopic (observed, empirical, or phenomenological) rate of 3 reaction v to cA the concentration of one of the chemical species present, as defined by4

5 𝛼 = ( ∂ln {𝑣}

∂ln {cA})[B],…

6 The argument in the ln function should be of dimension 1. Thus, reduced quantities 7 should be used, i.e., the quantity divided by its unit, {v} = v/(mol dm-3 s-1) and = {𝑐A}8 cA/(mol dm-3).9 Note 1: A rate equation can often be expressed in the form v = k [A][B]... ,

10 describing the dependence of the rate of reaction on the concentrations [A], [B],..., 11 where exponents , , ... are independent of concentration and time and k is 12 independent of [A], [B], ... . In this case the reaction is said to be of order with respect 13 to A, of order with respect to B,..., and of (total or overall) order n = + +... . The 14 exponents , ,... sometimes called "partial orders of reaction", can be positive or 15 negative, integral, or rational nonintegral numbers.16 Note 2: For an elementary reaction a partial order of reaction is the same as the 17 stoichiometric number. The overall order is then the same as the molecularity. For 18 stepwise reactions there is no general connection between stoichiometric numbers and 19 partial orders. Such reactions may have more complex rate laws, so that an apparent 20 order of reaction may vary with the concentrations of the chemical species involved and 21 with the progress of the reaction: in such cases it is not useful to speak of orders of 22 reaction, although apparent orders of reaction may be deducible from initial rates.23 Note 3: In a stepwise reaction, orders of reaction may in principle be assigned to the 24 elementary steps.25 Note 4: For chemical rate processes occurring in systems for which concentration

26 changes are not measurable, as in the case of a dynamic equilibrium aA ⇄ pP, and if a

27 chemical flux –A is found experimentally (e.g., by NMR line-shape analysis) to be 28 related to the concentration of A and to concentrations of other species B, ..., by the 29 equation3031 –A = k[A][B] ...3233 then the reaction is of order α with respect to A... and of total (or overall) order 34 (= + +...). 35 Note 5: If the overall rate of reaction is given by3637 v = k[A][B]

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12 but [B] remains constant in any particular sample (but can vary from sample to sample), 3 then the order of the reaction in A will be, and the rate of disappearance of A can be 4 expressed in the form56 vA = kobs[A]78 The proportionality factor kobs is called the "observed rate coefficient" and is related to 9 the rate constant k by

1011 kobs = k[B]

1213 Note 6: For the frequent case where = 1, kobs is often referred to as a "pseudo-first-14 order rate coefficient".15 See [13].16 See also kinetic equivalence, rate coefficient, rate constant.17 rev[3]18192021 organocatalysis22 Catalysis by small organic molecules, as distinguished from catalysis by (transition) 23 metals or enzymes. 24 Note 1: Frequently used organocatalysts are secondary amines (covalent catalysis 25 by generation of enamines or of iminium ions as reactive intermediates) and thioureas 26 (hydrogen-bonding catalysis). 27 Note 2: Organocatalysts are often employed for enantioselectivity.28 Note 3: The mechanisms employed by organocatalysts are examples of general acid 29 catalysis, general base catalysis, nucleophilic catalysis, specific acid catalysis, specific 30 base catalysis.31 See [339,340,341,342].3233 outer-sphere electron transfer34 Feature of an electron transfer between redox centres not sharing a common atom or 35 group.36 Example:37 MnO4

– + Mn*O42– ⇄ MnO4

2– + Mn*O4–

38 Note 1: In the transition state the interaction between the relevant electronic orbitals 39 of the two centres is weak (below 20 kJ mol–1), and the electron(s) must tunnel through 40 space.

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1 Note 2: If instead the donor and the acceptor exhibit a strong electronic coupling, 2 often through a ligand that bridges both, the reaction is described as inner-sphere 3 electron transfer. 4 Note 3: These two terms derive from studies of metal complexes, and for organic 5 reactions the terms "nonbonded' and 'bonded" electron transfer are often used.6 See [9,182,343].7 rev[3]89 oxidation

10 (1) Removal of one or more electrons from a molecular entity. 11 (2) Increase in the oxidation number of an atom within a substrate, see [344].12 (3) Gain of oxygen and/or loss of hydrogen by an organic substrate.13 Note 1: All oxidations meet criterion (2) and many meet criterion (3), but this is not 14 always easy to demonstrate. Alternatively, an oxidation can be described as the 15 transformation of an organic substrate by removal of one or more electrons from the 16 substrate, often accompanied by gain or loss of water, hydrons, and/or hydroxide, or by 17 nucleophilic substitution, or by molecular rearrangement.18 Note 2: This formal definition allows the original idea of oxidation (combination with 19 oxygen), together with its extension to removal of hydrogen, as well as processes 20 closely akin to this type of transformation (and generally regarded in organic chemistry 21 to be oxidations and to be effected by "oxidizing agents") to be descriptively related to 22 definition (1). For example the oxidation of methane to chloromethane may be 23 considered as24 CH4 – 2e– – H+ + HO– = CH3OH (oxidation), 25 followed by 26 CH3OH + HCl = CH3Cl + H2O (neither oxidation nor reduction)27 rev[3]2829 oxidation number30 oxidation state31 Number assigned to a carbon atom in a covalent organic compound according to3233 NOx = NX + NO + NN – NH – NM

3435 where NX, NO, NN, NH, and NM are the numbers of bonds to halogen, oxygen (or sulfur), 36 nitrogen, hydrogen, and a metal, respectively.37 Examples:3839 CH3MgBr (-4), CH3CH3 (-3), CH2=CH2 (-2), CH3OH (-2), CH2Cl2 (0),40 CH3CH=O (+1), HCN (+2), CCl4 (+4).

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12 Note 1: This assignment is based on the convention that each attached atom more 3 electronegative than carbon contributes +1, while each atom less electronegative 4 (including H) contributes -1, and an attached carbon contributes zero.5 Note 2: Oxidation numbers are not significant in themselves, but changes in oxidation 6 number are useful for recognizing whether a reaction is an oxidation or a reduction or 7 neither.8 Note 3: A different system is used for transition-metal species [345].9 See also electronegativity, oxidation.

10 rev[3]1112 oxidative addition13 Insertion of the metal of a metal complex into a covalent bond involving formally an 14 overall two-electron loss on one metal or a one-electron loss on each of two metals, 15 i.e.,

16 LnMm + XY → LnMm+2(X)(Y)

17 or 2 LnMm + XY → LnMm+1(X) + LnMm+1(Y)18 rev[3]1920 oxidative coupling21 Coupling of two molecular entities through an oxidative process, usually catalysed by a 22 transition-metal compound.23 Example (where the oxidant can be O2 or Cu(II) or others):24

2526 rev[3]2728 parallel effect29 Change of the position of the transition state upon stabilization or destabilization of a 30 structure (or structures) along the assumed minimum-energy reaction path.31 See Hammond postulate, More O'Ferrall - Jencks diagram.3233 parallel reaction34 See composite reaction.35 [3]3637 paramagnetism

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1 Property of substances having a magnetic susceptibility greater than 0, whereby they 2 are drawn into a magnetic field.3 See also diamagnetism.4 [3]56 partial rate factor pf

Z, mfZ

7 Rate constant for substitution at one specific site in an aromatic compound divided by 8 the rate constant for substitution at one position in benzene.9 Note 1: The partial rate factor pf

Z for para-substitution in a monosubstituted benzene 10 C6H5Z is related to the rate constants k(C6H5Z) and k(C6H6) for the total reactions (i.e., 11 at all positions) of C6H5Z and benzene, respectively, and fpara (the fraction of para-12 substitution in the total product formed from C6H5Z, usually expressed as a percentage) 13 by the relation14

15 𝑝Z𝑓 =

6𝑘(C6H5Z)𝑘(C6H6) 𝑓𝑝𝑎𝑟𝑎

1617 Similarly for meta-substitution:18

19 𝑚Z𝑓 =

6𝑘(C6H5Z)2𝑘(C6H6) 𝑓𝑚𝑒𝑡𝑎

2021 The symbols fp

Z, fmZ, fo

Z are also in use.22 Note 2: The term applies also to the ipso position, and it can be extended to other 23 substituted substrates undergoing parallel reactions at different sites with the same 24 reagent according to the same rate law.25 See [262,346,347].26 See also selectivity.27 rev[3]2829 partition ratio (partition constant, distribution ratio) P 30 Concentration of a substance in one phase divided by its concentration in another 31 phase, at equilibrium.32 Example, for an aqueous/organic system the partition ratio (or distribution ratio D) is 33 given by34 P = corg(A)/caq(A)35 Note 1: The most common way of applying P in correlation analysis or quantitative 36 structure-activity relationships is as lg P.37 Note 2: The parameter P is extensively used as an indicator of the capacity of a 38 molecular entity to cross biological membranes by passive diffusion.

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1 Note 3: The term partition coefficient is in common usage in toxicology but is not 2 recommended for use in chemistry and should not be used as a synonym for partition 3 constant, partition ratio, or distribution ratio. 4 See [167,232].5 See also Hansch constant, octanol-water partition ratio.67 pericyclic reaction8 Chemical reaction in which concerted reorganization of bonding takes place throughout 9 a cyclic array of continuously bonded atoms.

10 Note 1: It may be viewed as a reaction proceeding through a fully conjugated cyclic 11 transition state.12 Note 2: The term embraces a variety of processes, including cycloaddition, 13 cheletropic reaction, electrocyclic reaction and sigmatropic rearrangement, etc. 14 (provided they are concerted).15 See also pseudopericyclic.16 [3]1718 permittivity, relative εr

19 dielectric constant (obsolete)20 Measure of the reduction of the magnitude of the potential energy of interaction between 21 two charges on going from vacuum to a condensed medium, expressed as the ratio of 22 the former to the latter.23 Note: The term dielectric constant is obsolete. Moreover, the dielectric constant is 24 not a constant since it depends on frequency.25 See [12].2627 perpendicular effect28 Change of the position of the transition state upon stabilization or destabilization of a 29 structure (or structures) that lies off the assumed minimum-energy reaction path.30 See anti-Hammond effect, Hammond postulate, More O'Ferrall - Jencks diagram.31 rev[3]3233 persistence34 Characteristic of a molecular entity that has an appreciable lifetime (minutes or 35 nanoseconds or other, depending on context).36 Note 1: Dilute solution or inert solvent may be required for persistence.37 Note 2: Persistence is a kinetic or reactivity property, whereas, in contrast, stability 38 (being stable) is a thermodynamic property.39 See [348].40 See also transient.

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1 rev[3]23 pH-rate profile4 Plot of observed rate coefficient, or more usually the decadic logarithm of its numerical 5 value, against solution pH, other variables being kept constant.6 [3]78 phase-transfer catalysis9 Enhancement of the rate of a reaction between chemical species located in different

10 phases (immiscible liquids or solid and liquid) by addition of a small quantity of an agent 11 (called the phase-transfer catalyst) that extracts one of the reactants, most commonly 12 an anion, into the other phase so that reaction can proceed.13 Note 1: These catalysts are often onium ions (e.g., tetraalkylammonium ions) or 14 complexes of inorganic cations (e.g., as crown ether complexes).15 Note 2: The catalyst cation is not consumed in the reaction although an anion 16 exchange does occur.17 Example:18 CH3(CH2)7Cl in decane + aqueous Na+CN– + catalytic PhCH2N(CH2CH3)3

+ 19 = CH3(CH2)7CN + aqueous Na+Cl–20 rev[3]2122 phenonium ion23 See bridged carbocation.24 [3]2526 photochromism27 Reversible transformation of a molecular entity between two forms, A and B, having 28 different absorption spectra, induced in one or both directions by absorption of 29 electromagnetic radiation.30 Note 1: The thermodynamically stable form A is transformed by irradiation into form 31 B. The back reaction can occur thermally (photochromism of type T) or photochemically 32 (photochromism of type P).33 Note 2: The spectral change is typically, but not necessarily, of visible colour.34 Note 3: An important parameter is the number of cycles that a photochromic system 35 can undergo.36 See [9].3738 photolysis39 Bond cleavage induced by ultraviolet, visible, or infrared radiation. 40 Example:

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1 Cl2 → 2 Cl2 Note: The term is used incorrectly to describe irradiation of a sample without any 3 bond cleavage, although in the term “flash photolysis” this usage is accepted.4 See [9].5 [3]67 photostationary state8 Steady state reached by a chemical system undergoing photochemical reaction, such 9 that the rates of formation and disappearance are equal for each of the transient

10 molecular entities formed.11 See [9].1213 pi-adduct14 See -adduct.1516 pi-bond17 See , .181920 polar aprotic solvent21 See dipolar non-HBD solvent.22 [3]2324 polar effect25 All the interactions whereby a substituent on a reactant molecule RY modifies the 26 electrostatic forces operating at the reaction centre Y, relative to the reference standard 27 RoY.28 Note 1: These forces may be governed by charge separations arising from 29 differences in the electronegativity of atoms (leading to the presence of dipoles), by the 30 presence of monopoles, or by electron delocalization.31 Note 2: It is distinguished from a steric effect.32 Note 3: Sometimes, however, the term polar effect is taken to refer to the influence, 33 other than steric, that non-conjugated substituents exert on reaction rates or equilibria, 34 thus excluding effects of electron delocalization between a substituent and the 35 molecular framework to which it is attached. 36 See also electronic effects (of substituents), field effect, inductive effect.37 [3]3839 polar solvent

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1 Liquid composed of molecules with a significant dipole moment, capable of dissolving 2 ions or other molecules with significant dipole moments.3 See polarity.4 rev[3]56 polarity (of a bond)7 Characteristic of a bond between atoms of different electronegativity, such that the 8 electrons in that bond are not shared equally.9

10 polarity (of a solvent)11 Overall solvation capability (solvation power) of a solvent toward solutes, which 12 depends on the action of all possible intermolecular interactions between solute ions or 13 molecules and solvent molecules, excluding interactions leading to definite chemical 14 alterations of the ions or molecules of the solute.15 Note: Quantitative measures of solvent polarity include relative permittivity (dielectric 16 constant) and various spectroscopic parameters.17 See [17].18 See solvent parameter.19 rev[3]2021 polarizability22 23 (SI unit: C m2 V–1)24 electric polarizability 25 Induced dipole moment, induced, divided by applied electric field strength E26 = induced/E 27 Note 1: The polarizability represents the ease of distortion of the electron cloud of a 28 molecular entity by an electric field (such as that due to the proximity of a charged 29 species).30 Note 2: Polarizability is more often expressed as polarizability volume, with unit cm3, 31 where 0 is the permittivity of vacuum:32

33 /cm3 = 106

4π𝜀0

d𝝁d𝐸

3435 Note 3: In general, polarizability is a tensor that depends on direction, for example, 36 depending on whether the electric field is along a bond or perpendicular to it, and the 37 induced dipole may not even be along the direction of the electric field. However, in 38 ordinary usage the term refers to the mean polarizability, the average over three 39 rectilinear axes of the molecule.

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1 rev[3]23 polydent4 polydentate5 See ambident.6 [3]78 potential-energy profile9 Curve describing the variation of the potential energy of a system of atoms as a function

10 of a single coordinate.11 Note 1: For an elementary reaction the relevant coordinate is the reaction coordinate, 12 which is a measure of progress along the minimum-energy reaction path (MERP) from 13 a saddle point on a potential-energy surface in each direction toward adjacent energy 14 minima. For a stepwise reaction it is the succession of reaction coordinates for the 15 successive individual reaction steps. For a reaction involving a bifurcation each branch 16 requires a different reaction coordinate and has its own profile.17 Note 2: A profile constructed as a function of an arbitrary internal coordinate (for 18 example, a bond distance) is not guaranteed to pass through a saddle point on the 19 corresponding potential-energy surface; in order to do so, it must be smooth, 20 continuous, and follow the path of lowest energy connecting the reactant and product 21 energy minima.22 Examples: (one-step reaction, two-step reaction)23

242526 See [349].27 See also Gibbs energy diagram, potential-energy surface.28 rev[3]2930 potential-energy surface (PES)31 Surface describing the variation within the Born-Oppenheimer approximation of the 32 potential energy of a system of atoms as a function of a set of internal coordinates.

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1 Note 1: A minimum on a PES is characterized by positive curvature in all directions 2 and corresponds to a structure that is stable with respect to small displacements away 3 from its equilibrium geometry; for this structure (a reactant, intermediate or product) all 4 vibrational frequencies are real. A saddle point is characterized by positive curvature in 5 all directions except for one with negative curvature and corresponds to a transition 6 structure, for which one vibrational frequency is imaginary. A local maximum is 7 characterized by negative curvatures in two (or more) directions and has two (or more) 8 imaginary frequencies; it is sometimes called a second-order (or higher-order) saddle 9 point.

10 Note 2: It is usual to select only two coordinates in order to represent the surface, 11 with potential energy as the third dimension, or alternatively as a two-dimensional 12 contour map. For example, a PES for a simple reacting triatomic system A–B + C → 13 A + B–C, could be constructed using the A···B and B···C distances as two internal 14 coordinates; the third independent coordinate could be the ABC angle or the A···C 15 distance, and its value could either be kept fixed or else be relaxed to minimize the 16 energy at each point on the (A···B, B···C) surface.17 Note 3: The path of steepest-descent from a saddle point in each direction towards 18 adjacent energy minima defines a minimum-energy reaction path (MERP) that is 19 equivalent to the energetically easiest route from reactants to products. The change in 20 potential energy along this path across the PES defines a potential-energy profile for 21 the elementary reaction. Progress along this path is measured by the value of the 22 reaction coordinate.23 Note 4: In general there is neither a unique set of internal coordinates nor a unique 24 choice of two coordinates with which to construct a PES of reduced dimensionality. 25 Consequently there is no guarantee of a smooth and continuous PES containing a 26 saddle point connecting reactant and product minima. 27 See [8,73,350,351].28 See: bifurcation, minimum-energy reaction path, potential-energy profile, reaction 29 coordinate, transition structure.30 rev[3]3132 potential of mean force (PMF)33 Free energy as a function of a set of coordinates, the negative gradient of which gives 34 the average force acting on that configuration averaged over all other coordinates and 35 momenta within a statistical distribution. If the averaging is performed within a canonical 36 ensemble (constant volume, temperature, and number of particles) the PMF is 37 equivalent to the Helmholtz energy, but if it is performed within an isobaric-isothermal 38 ensemble (constant pressure, temperature, and number of particles) the PMF is 39 equivalent to the Gibbs energy.

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1 Note 1: Commonly, the PMF acting upon a selected geometric variable and averaged 2 over the coordinates and momenta of all other geometric variables is evaluated for a 3 succession of constrained values of the selected variable, thereby generating 4 (generically) a free-energy profile with respect to the selected reaction coordinate (e.g., 5 a bond distance or angle, or a combination of internal coordinates); specifically this is 6 either a Helmholtz-energy or a Gibbs-energy profile, depending upon the choice of 7 ensemble for the statistical averaging within a computational simulation. 8 Note 2: Selection of two geometric variables as reaction coordinates allows a free-9 energy surface to be computed as a two-dimensional PMF.

10 Note 3: Molecular simulations often yield Helmholtz energies, not Gibbs energies, 11 but for condensed phases the difference is usually neglected.12 See: free energy, reaction coordinate.1314 pre-association15 Step on the reaction path of some stepwise reactions in which the molecular entity C 16 forms an encounter pair or encounter complex with A prior to the reaction of A to form 17 product.18 Example:

192021 Note 1: In this mechanism the chemical species C may but does not necessarily 22 assist the formation of B from A, which may itself be a bimolecular reaction with some 23 other reagent.24 Specific example (aminolysis of phenyl acetate):25

2627

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1 Note 2: Pre-association is important when B is too short-lived to permit B and C to 2 come together by diffusion. In the specific example, T± would dissociate faster than 3 general acid HA can diffuse to it. Experimentally, the Brønsted α is > 0, which is 4 inconsistent with rate-limiting diffusion and hydron transfer.5 See [352].6 See also Brønsted relation, microscopic diffusion control, spectator mechanism.7 rev[3]89 precursor complex

10 See encounter complex.11 [3]1213 pre-equilibrium14 prior equilibrium15 Rapid reversible step preceding the rate-limiting step in a stepwise reaction.16 Example:

171819 See also kinetic equivalence, steady state.20 [3]2122 pre-exponential factor23 See energy of activation, entropy of activation.24 [3]2526 principle of nonperfect synchronization27 Consideration applicable to reactions in which there is a lack of synchronization 28 between bond formation or bond rupture and other changes that affect the stability of 29 products and reactants, such as resonance, solvation, electrostatic, hydrogen bonding 30 and polarizability effects. 31 Note: The principle states that a product-stabilizing factor whose development lags 32 behind bond changes at the transition state, or a reactant-stabilizing factor whose loss 33 is ahead of bond changes at the transition state, increases the intrinsic barrier and 34 decreases the rate constant of a reaction. For a product-stabilizing factor whose 35 development is ahead of bond changes, or a reactant-stabilizing factor whose loss lags 36 behind bond changes, the opposite relations hold. The reverse effects are observable 37 for factors that destabilize a reactant or product. 38 See [108].

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1 See also imbalance, synchronous.23 prior equilibrium4 See pre-equilibrium.5 [3]67 product-determining step8 Step of a stepwise reaction in which the product distribution is determined.9 Note: The product-determining step may be identical to, or may occur later than, the

10 rate-determining step in the reaction.11 [3]1213 product-development control14 Case of kinetic control in which the selectivity of a reaction parallels the relative 15 (thermodynamic) stabilities of the products.16 Note: Product-development control arises because whatever effect stabilizes or 17 destabilizes a product is already operative at the transition state. Therefore it is usually 18 associated with a transition state occurring late on the minimum-energy reaction path.19 See also steric-approach control, thermodynamic control.20 [3]2122 promotion23 See pseudo-catalysis.24 [3]2526 propagation27 See chain reaction.28 [3]2930 propargylic substitution31 See allylic substitution reaction.3233 protic34 See protogenic.35 [3]3637 protic solvent38 Solvent that is capable of acting as a hydrogen-bond donor.39 See HBD solvent.40

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1 protofugality2 special case of nucleofugality, describing the relative rates of transfer of a proton (more 3 generally: hydron) from a series of Brønsted acids H–X to a common Brønsted base. 4 Note: This term has the advantage over the commonly used “kinetic acidity” that 5 philicity and fugality are associated with kinetics, while acidity and basicity are 6 associated with thermodynamics.7 See [194].8 See also Brønsted acidity, nucleofugality, protophilicity. 9

10 protogenic (solvent)11 HBD (hydrogen bond donor) solvent.12 Capable of acting as a proton (hydron) donor.13 Note 1: Such a solvent may be a strong or weak Brønsted acid.14 Note 2: The term is preferred to the synonym protic or to the more ambiguous 15 expression acidic.16 See protophilic solvent.17 [3]1819 protolysis20 proton (hydron)-transfer reaction.21 Note: Because of its misleading similarity to hydrolysis, photolysis, etc., this use is 22 discouraged.23 See also autoprotolysis.24 rev[3]2526 proton affinity27 Negative of the enthalpy change in the gas phase reaction between a proton (more 28 appropriately hydron) and the chemical species concerned, usually an electrically 29 neutral or anionic species, to give the conjugate acid of that species.30 Note 1: For an anion A–, the proton affinity is the negative of the enthalpy of the 31 heterolytic dissociation (in the gas phase) of the Brønsted acid HA.32 Note 2: Proton affinity is often, but unofficially, abbreviated as PA.33 Note 3: Affinity properly refers to Gibbs energy.34 See also gas phase basicity, gas phase acidity.35 See [2]. See also [213,353].36 rev[3]3738 protonation39 Attachment of the ion 1H+ (of relative atomic mass 1).40 See also hydronation.

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12 proton–transfer reaction3 Chemical reaction, the main feature of which is the intermolecular or intramolecular 4 transfer of a proton (hydron) from one binding site to another.5 Example:6 CH3COOH + (CH3)2C=O → CH3CO2

– + (CH3)2C=OH+

7 Note: In the detailed description of proton-transfer reactions, especially of rapid 8 proton transfers between electronegative atoms, it should always be specified whether 9 the term is used to refer to the overall process (including the more-or-less encounter-

10 controlled formation of a hydrogen-bonded complex and the separation of the products) 11 or just to the proton-transfer event (including solvent rearrangement) by itself.12 See also autoprotolysis, microscopic diffusion control, tautomerism.13 [3]1415 protophilic (solvent)16 See HBA (hydrogen bond acceptor) solvent.17 rev[3]18192021 protophilicity22 Special case of nucleophilicity, describing the relative rates of reactions of a series of 23 Lewis bases with a common Brønsted acid. This term has the advantage over the 24 commonly used “kinetic basicity” that philicity and fugality are associated with kinetics, 25 while acidity and basicity are associated with thermodynamics.26 See [194].27 See also Brønsted basicity, nucleophilicity, protofugality.2829 prototropic rearrangement (prototropy)30 See tautomerization.31 [3]3233 pseudo-catalysis34 Increase of the rate of a reaction by an acid or base present in nearly constant 35 concentration throughout a reaction in solution (owing to buffering or to the use of a 36 large excess), even though that acid or base is consumed during the process, so that 37 the acid or base is not a catalyst and the phenomenon strictly cannot be called catalysis 38 according to the established meaning of these terms in chemical kinetics.39 Note 1: Although the mechanism of such a process is often closely related to that of 40 a catalysed reaction, it is recommended that the term pseudo-catalysis be used in these

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1 and analogous cases. For example, if a Brønsted acid accelerates the hydrolysis of an 2 ester to a carboxylic acid and an alcohol, this is properly called acid catalysis, whereas 3 the acceleration, by the same acid, of the hydrolysis of an amide should be described 4 as pseudo-catalysis because the acid pseudo-catalyst is stoichiometrically consumed 5 during the reaction through formation of an ammonium ion.6 Note 2: The terms general-acid pseudo-catalysis and general-base pseudo-catalysis 7 may be used as the analogues of general acid catalysis and general base catalysis.8 Note 3: The terms acid- and base-promoted, acid- and base-accelerated, and acid- 9 and base-induced are sometimes used for reactions that are pseudo-catalysed by or

10 bases. However, the term promotion also has a different meaning in other chemical 11 contexts.12 rev[3]1314 pseudo-first-order rate coefficient15 See order of reaction, rate coefficient.16 [3]1718192021 pseudopericyclic22 Feature of a concerted transformation in which the primary changes in bonding occur 23 within a cyclic array of atoms but in which one (or more) nonbonding and bonding 24 atomic orbitals interchange roles.25 Examples: enol-to-enol tautomerism of 4-hydroxypent-3-en-2-one (where the 26 electron pairs in the O–H bond and in the lone pair on the other O are in σ orbitals 27 whereas the other three electron pairs are in orbitals) and hydroboration (where the 28 B uses an sp2 bonding orbital and a vacant p orbital)29

303132 Note: Because the atomic orbitals that interchange roles are orthogonal, such a 33 reaction does not proceed through a fully conjugated transition state and is thus not a 34 pericyclic reaction. It is therefore not governed by the rules that express orbital 35 symmetry restrictions applicable to pericyclic reactions.36 See [354,355].37 See also coarctate.38 rev[3]

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12 push-pull conjugation3 Feature of an extended conjugated system bearing an electron donor group at one 4 end and an electron acceptor group at the other end.5 See [356].6 See also cross-conjugation.78 pyrolysis9 Thermolysis, usually associated with exposure to a high temperature.

10 See also flash vacuum pyrolysis.11 [3]1213 -adduct (pi-adduct)14 Adduct formed by electron-pair donation from a orbital into a * orbital, or from a 15 orbital into a * orbital, or from a orbital into a * orbital. 16 Example:

1718 Note: Such an adduct has commonly been known as a complex, but, as the 19 bonding is not necessarily weak, it is better to avoid the term complex, in accordance 20 with the recommendations in this Glossary.21 See also coordination.22 [3]2324 -bond (pi bond)25 Interaction between two atoms whose p orbitals overlap sideways.26 Note: The designation as is because the p orbitals are antisymmetric with respect 27 to a defining plane containing the two atoms. 28 See sigma, pi.29 rev[3]3031 -complex32 See -adduct.33 [3]34

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1 -electron acceptor2 Substituent capable of electron withdrawal by resonance (e.g., NO2).3 See electronic effect, polar effect, -electron donor, -constant.4 rev[3]56 -electron donor7 Substituent capable of electron donation by resonance (e.g., OCH3).8 See electronic effect, polar effect, -electron acceptor, -constant.9 rev[3]

1011 quantitative structure-activity relationship (QSAR)12 Regression model to correlate biological activity or chemical reactivity with predictor 13 parameters based on measured or calculated features of molecular structure.14 See [357,358].15 See also correlation analysis.16 rev[3]1718 quantitative structure-property relationship (QSPR)19 Regression model to correlate chemical properties such as boiling point or 20 chromatographic retention time with predictor parameters based on measured or 21 calculated features of molecular structure.22 See [359].23 See also correlation analysis.2425 quantum yield26 Number of defined events that occur per photon absorbed by the system.27 Note 1: The integral quantum yield is the number of events divided by the number 28 of photons absorbed in a specified wavelength range. 29 Note 2: For a photochemical reaction () is the amount of reactant consumed or 30 product formed divided by the number of photons absorbed at wavelength.31 Note 3: The differential quantum yield for a homogeneous system is32

33 𝛷(𝜆) = |d𝑥d𝑡|

𝑞𝑝,𝜆[1 ― 10𝐴(𝜆)]3435 where |dx/dt| is the rate of change of a quantity x that measures the progress of a

36 reaction, qp, is the spectral photon flux (mol or its non-SI equivalent einstein) incident

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1 per unit time at wavelength , and A() is the decadic absorbance at the excitation 2 wavelength.3 Note 4: When the quantity x is an amount concentration, it is convenient to use in 4 the denominator the rate (in moles) of photons absorbed per volume. 5 See [9,10].6 rev[3]78 radical9 free radical (obsolete)

10 Molecular entity possessing an unpaired electron.11 Examples: •CH3, •SnR3, Cl•12 Note 1: In these formulae the dot, symbolizing the unpaired electron, should be 13 placed so as to indicate the atom of highest spin density, if possible.14 Note 2: Paramagnetic metal ions are not normally regarded as radicals. However, in 15 the isolobal analogy the similarity between certain paramagnetic metal ions and radicals 16 becomes apparent.17 Note 3: Depending upon the core atom that possesses the highest spin density, the 18 radicals can be described as carbon-, oxygen-, nitrogen-, or metal-centred radicals.19 Note 4: If the unpaired electron occupies an orbital having considerable s or more or 20 less pure p character, the respective radicals are termed or radicals.21 Note 5: The term radical has also been used to designate a substituent group within 22 a molecular entity, as opposed to "free radical", which is now simply called radical. The 23 bound entities may be called groups or substituents, but should no longer be called 24 radicals.25 See [40,97].26 See also diradical.27 rev[3]2829 radical combination30 Formation of a covalent bond by reaction of one radical with another.31 See colligation.32 rev[3]3334 radical ion35 Radical that carries a net electric charge.36 Note 1: A positively charged radical is called a radical cation (e.g., the benzene 37 radical cation C6H6

•+); a negatively charged radical is called a radical anion (e.g., the 38 benzene radical anion C6H6

•– or the benzophenone radical anion Ph2C–O•–).39 Note 2: Unless the positions of unpaired spin and charge can be associated with 40 specific atoms, superscript dot and charge designations should be placed in the order

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1 •+ or •–, as suggested by the name radical ion. However, the usage in mass 2 spectrometry is to place the charge symbol before the dot [7].3 Note 3: In the first edition of this Glossary it was recommended to place the charge 4 designation directly above the dot. This format is now discouraged because of the 5 difficulty of extending it to ions bearing more than one charge and/or more than one 6 unpaired electron.7 [3]89 radical pair

10 geminate pair11 Two radicals in close proximity in solution, within a solvent cage.12 Note 1: The two radicals may be formed simultaneously by some unimolecular 13 process, e.g., peroxide decomposition or photolysis, or they may have come together 14 by diffusion.15 Note 2: While the radicals are together, correlation of the unpaired electron spins of 16 the two species cannot be ignored: this correlation is responsible for the CIDNP 17 phenomenon.18 See also geminate recombination.19 See [9,40].20 [3]2122 radiolysis23 Cleavage of one or several bonds resulting from exposure to high-energy radiation.24 Note: The term is also often used loosely to specify the method of irradiation (e.g., 25 pulse radiolysis) used in any radiochemical reaction, not necessarily one involving bond 26 cleavage.27 [3]2829 rate coefficient30 Empirical constant k in the equation for the rate of a reaction that is expressible by an 31 equation of the form3233 v = k[A]a[B]b....3435 Note 1: It is recommended that the term rate constant be confined to reactions that 36 are believed to be elementary reactions.37 Note 2: When a rate coefficient relates to a reaction occurring by a composite 38 mechanism, it may vary not only with temperature and pressure but also with the 39 concentration of reactants. For example, in the case of a unimolecular gas reaction the 40 rate at sufficiently high pressures is given by

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12 v = k[A]34 whereas at low pressures the rate expression is56 v = k'[A]278 Similarly, for a second-order reaction, the rate is given by9

10 v = k2[A][B]1112 But under conditions where [B] remains constant at [B]0, as when B is a catalyst or is 13 present in large excess, 1415 v = k2[A][B]0 = k[A]1617 where the rate coefficient k = k2[B]0, which varies with [B]0.18 Such a rate coefficient k, which varies with the concentration [B], is called a first-19 order rate coefficient for the reaction, or a pseudo first-order rate constant even though 20 it is not a rate constant.21 Note: Rate constant and rate coefficient are often used as synonyms, see [12], 22 section 2.12, p.63.23 See [13].24 See order of reaction.25 rev[3]2627 rate constant28 k29 Term generally used for the rate coefficient of a reaction that is believed to be an 30 elementary reaction. See [13].31 Note 1: In contrast to a rate coefficient, a rate constant should be independent of 32 concentrations, but in general both rate constant and rate coefficient vary with 33 temperature.34 Note 2: Rate constant and rate coefficient are often used as synonymous, see [12], 35 section 2.12, p.63.36 See order of reaction.37 rev[3]3839 rate-controlling step 40 See rate-determining states, rate-limiting step.

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1 rev[3]23 rate law4 empirical differential rate equation5 Expression for the rate of reaction in terms of concentrations of chemical species and 6 constant parameters (normally rate coefficients and partial orders of reaction) only.7 For examples of rate laws see the equations under kinetic equivalence, and under 8 steady state.9 [3]

1011 rate-limiting step12 rate-controlling step13 rate-determining step14 Step in a multistep reaction that is the last step in the sequence whose rate constant 15 appears in the rate equation.16 See [13].17 Example: Two-step reaction of A with B to give intermediate C, which then reacts 18 further with D to give products:

1920 If [C] reaches a steady state, then the observed rate is given by2122 v = –d[A]/dt = k1k2[A][B][D]/(k–1 + k2[D])2324 Case 1: If k2[D] >> k–1,then the observed rate simplifies to2526 v = –d[A]/dt = k1[A][B]2728 Because k2 disappears from the rate equation and k1 is the last rate constant to remain, 29 step (1) is said to be rate-limiting.30 Case 2: If k2[D] << k–1, then the observed rate is given by3132 v = k1k2[A][D]/k–1 = Kk2[A][B][D]3334 where K, equal to k1/k–1, is the equilibrium constant for the pre-equilibrium (Step 1). 35 Because k2 remains in the rate equation, Step 2 is said to be rate-limiting. Notice that 36 in this case, where Step 2 involves another reactant D, which step is rate-limiting can 37 depend on [D]: Step 1 at high [D] and Step 2 at low [D].

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12 Specific example: Acid-catalyzed bromination of a methyl ketone3

456 where k1 = k1'[H+], k–2 = k–2'[H+], k3 = k3'[Br2], k–3 = k–3'[Br–].78 According to the steady-state approximation,9

10 v = kobs[RCOCH3] = k1k2k3k4[RCOCH3]/{k2k3k4 + k–1k3k4 + k–1k–2(k–3 + k4)}.1112 or kobs = k1k2k3k4/{k2k3k4 + k–1k3k4 + k–1k–2(k–3 + k4)}.1314 If k4 >> k–3, k–3 can be ignored in the parentheses, so that kobs simplifies to 15 k1k2k3k4/{k2k3k4 + k–1k3k4 + k–1k–2k4}. Then if k3 >> k–2, kobs simplifies further to 16 k1k2k3k4/{k2k3k4 + k–1k3k4}, where k3k4 cancels and kobs becomes k1k2/{k–1 + k2}, so that 17 k2 is the last rate constant remaining in kobs. The second step is rate-limiting, and the 18 rate is independent of k3 or of [Br2].19 Note 1: Although the expressions rate-controlling, rate-determining, and rate-limiting 20 are often regarded as synonymous, rate-limiting is to be preferred, because in Case 2 21 all three rate constants enter into the rate equation, so that all three are rate-controlling 22 and rate-determining, but the first step is not rate-limiting. 23 Note 2: If the concentration of any intermediate builds up to an appreciable extent, 24 then the steady-state approximation no longer holds, and the reaction should be 25 analyzed as though that intermediate is the reactant.26 Note 3: It should be noted that a catalytic cycle does not have a rate-determining 27 step. Instead, under steady-state conditions, all steps proceed at the same rate 28 because the concentrations of all intermediates adjust so as to offset the differences in 29 the corresponding rate constants [360].30 See also Gibbs energy diagram, microscopic diffusion control, mixing control.31 rev[3]3233 rate of reaction34 v35 (unit: mol dm–3 s–1 or mol L–1 s–1)36 For the general chemical reaction3738 a A + b B = p P + q Q ...39

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1 occurring under constant-volume conditions, without an appreciable build-up of reaction 2 intermediates, the rate of reaction v is defined as3

4 𝑣 = ― 1𝑎

d[A]d𝑡 = ―

1𝑏

d[B]d𝑡 = ―

1𝑝

d[P]d𝑡 = ―

1𝑞

d[Q]d𝑡

56 where symbols inside square brackets denote concentrations (conventionally 7 expressed in unit mol dm–3). The symbols R and r are also used instead of v. It is 8 recommended that the unit of time be the second. 9 Example:

1011 Cr2O7

2– + 3 (CH3)2CHOH + 8 H+ = 2 Cr3+ + 3 (CH3)2C=O + 7 H2O12

13 𝑣 = ― d[Cr2O 2 ―

7 ]d𝑡 = ―

13

d[(CH3)2CHOH]d𝑡 =

12

d[𝐶𝑟3 + ]d𝑡 =

13

d[(CH3)2C = O]d𝑡

1415 Note: For a stepwise reaction this definition of rate of reaction will apply only if there 16 is no accumulation of intermediate or formation of side products. It is therefore 17 recommended that the term rate of reaction be used only in cases where it is 18 experimentally established that these conditions apply. More generally, it is 19 recommended that, instead, the terms rate of disappearance or rate of consumption of 20 A (i.e., –d[A]/dt) or rate of appearance of P (i.e., d[P]/dt) be used, depending on the 21 particular chemical species that is actually observed. In some cases reference to the 22 chemical flux observed may be more appropriate.23 See [13].24 See also chemical relaxation, lifetime, order of reaction.25 rev[3]2627 reaction coordinate28 Parameter that changes during the conversion of one (or more) reactant molecular 29 entities into one (or more) product molecular entities and whose value can be taken as 30 a measure of the progress along a minimum-energy reaction path.31 Note 1: The term "reaction coordinate" is often used to refer to a geometric variable 32 itself (typically a bond distance or bond angle, or a combination of distances and/or 33 angles) as well as (or instead of) the value of that variable. Although strictly incorrect, 34 this usage is very commonly encountered.35 Note 2: In cases where the location of the transition structure is unknown, an internal 36 coordinate of the system (e.g., a geometric variable or a bond order, or an energy gap 37 between reactant-like and product-like valence-bond structures) is often selected as a 38 reaction coordinate. A potential-energy profile obtained by energy minimization over

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1 other coordinates for a succession of fixed values of an arbitrary reaction coordinate is 2 not guaranteed to pass through the transition structure unless it is a continuous function 3 of that reaction coordinate. Similarly, a free-energy profile obtained as a potential of 4 mean force with respect to an arbitrary reaction coordinate is not guaranteed to pass 5 through the lowest-energy transition state.6 Note 3: “Reaction coordinate” is sometimes used as an undefined label for the 7 horizontal axis of a potential-energy profile or a Gibbs energy diagram.8 See [349,361]. 9 See also Gibbs energy diagram, potential-energy profile, potential-energy surface.

10 rev[3]1112 reaction path13 (1) Synonym for mechanism.14 (2) Trajectory on the potential-energy surface.15 rev[3]1617 reaction step18 Elementary reaction constituting one of the stages of a stepwise reaction in which a 19 reaction intermediate (or, for the first step, the reactants) is converted into the next 20 reaction intermediate (or, for the last step, the products) in the sequence of 21 intermediates between reactants and products.22 [3]2324 reactive intermediate25 intermediate2627 reactivity (n.), reactive (adj.)28 Kinetic property of a chemical species by which (for whatever reason) it has a different 29 rate constant for a specified elementary reaction than some other (reference) species.30 Note 1: The term has meaning only by reference to some explicitly stated or implicitly 31 assumed set of conditions. It is not to be used for reactions or reaction patterns of 32 compounds in general.33 Note 2: Term also used more loosely as a phenomenological description not 34 restricted to elementary reactions. When applied in this sense, the property under 35 consideration may reflect not only rate constants but also equilibrium constants.36 See also stable, unreactive, unstable.37 [3]3839 reactivity index

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1 Numerical quantity derived from quantum-mechanical model calculations or from a 2 linear Gibbs-energy relationship (linear free-energy relationship) that permits the 3 prediction or correlation of relative reactivities of different molecular sites.4 Note: Many indices are in use, based on a variety of theories and relating to various 5 types of reaction. The more successful applications have been to the substitution 6 reactions of conjugated systems, where relative reactivities are determined largely by 7 changes of -electron energy and -electron density.8 rev[3]9

10 reactivity-selectivity principle (RSP)11 Idea that the more reactive a reagent is, the less selective it is.12 Note: There are many examples in which the RSP is followed, but there are also 13 many counterexamples. Although the RSP is in accord with intuitive feeling, it is now 14 clear that selectivity can decrease, increase, or remain constant as reactivity increases, 15 so that the RSP is unreliable as a guide to reactivity.16 See [126,346,362,363,364,365].17 rev[3]1819 rearrangement20 See degenerate rearrangement, molecular rearrangement, sigmatropic 21 rearrangement.22 [3]2324 reduction 25 (1) Transfer of one or more electrons to a molecular entity, usually inorganic.26 (2) Decrease in the oxidation number of any atom within any substrate [344].27 (3) Loss of oxygen or halogen and/or gain of hydrogen of an organic substrate.28 See oxidation.29 rev[3]3031 reductive elimination32 Reverse of oxidative addition.33 [3]3435 regioselectivity (n.), regioselective (adj.)36 Property of a reaction in which one position of bond making or breaking occurs 37 preferentially over all other possible positions.38 Note 1: The resulting regioisomers are constitutional isomers. 39 Note 2: Reactions are termed completely (100 %) regioselective if the discrimination 40 is complete, or partially (x %) if the product of reaction at one site predominates over

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1 the product of reaction at other sites. The discrimination may also be referred to semi-2 quantitatively as high or low regioselectivity.3 Note 3: Historically the term was restricted to addition reactions of unsymmetrical 4 reagents to unsymmetrical alkenes.5 Note 4: In the past, the term regiospecificity was proposed for 100 % regioselectivity. 6 This terminology is not recommended, owing to inconsistency with the terms 7 stereoselectivity and stereospecificity.8 See [366,367].9 See also chemoselectivity.

10 rev[3]1112 Reichardt ET parameter13 See Dimroth-Reichardt ET(30) parameter.1415 relaxation16 Passage of a system that has been perturbed from equilibrium, by radiation excitation 17 or otherwise, toward or into thermal equilibrium with its environment.18 See [9].19 See also chemical relaxation.20 rev[3]2122 reorganization energy23 Gibbs energy required to distort the reactants (and their associated solvent molecules) 24 from their relaxed nuclear configurations to the relaxed nuclear configurations of the 25 products (and their associated solvent molecules). 26 Note 1: This approach was originally formulated for one-electron transfer reactions, 27 A + D → A–· + D+·, in the framework of the Marcus equation, assuming weak coupling 28 between the reactants [368].29 Note 2: Reorganization energy is not the same as distortion energy, which is the 30 energy required to distort the reactants to the nuclear configuration of the transition 31 state.32 Note 3: This approach has been extended to enzyme-catalysed reactions [369].33 Note 4: Marcus theory has been shown to be valid for some complex reactions 34 (cycloaddition, SN2), even though the weak-coupling assumption is clearly not valid. In 35 these cases the reorganization energy (in terms of activation strain) is counteracted by 36 stabilizing interactions (electrostatic and orbital) [166].37 See also distortion interaction model, intrinsic barrier, Marcus equation.38 rev[3]3940 resonance

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1 Representation of the electronic structure of a molecular entity in terms of contributing 2 Lewis structures.3 Note 1: Resonance means that the wavefunction is represented by mixing the 4 wavefunctions of the contributing Lewis structures.5 Note 2: The contributing Lewis structures are represented as connected by a double-6 headed arrow (↔), rather than by the double arrow (⇄) representing equilibrium 7 between species.8 Note 3: This concept is the basis of the quantum-mechanical valence-bond methods. 9 The resulting stabilization is linked to the quantum-mechanical concept of resonance

10 energy. The term resonance is also used to refer to the delocalization phenomenon 11 itself.12 Note 4: This term has a completely different meaning in physics.13 See [75,370].14 See also [371].15 rev[3]1617 resonance effect18 Experimentally observable influence (on reactivity, etc.) of a substituent through 19 electron delocalization to or from the substituent.20 See [126,173,191].21 See also inductive effect.22 rev[3]2324 resonance energy25 Difference in potential energy between the actual molecular entity and the contributing 26 Lewis structure of lowest potential energy.27 Note: The resonance energy cannot be measured experimentally, but only 28 estimated, since contributing Lewis structures are not observable molecular entities.29 See resonance.30 rev[3]3132 resonance form33 resonance structure, contributing structure, canonical form34 One of at least two Lewis structures,with fixed single, double, and triple bonds, that is 35 a contributing structure to the valence-bond wavefunction of a molecule that cannot be 36 described by a single Lewis structure.37 Note 1: Although the valence-bond wavefunction is a linear combination of the 38 wavefunctions of the individual resonance forms, the coefficients and the relative 39 contributions of the various resonance forms are usually kept qualitative. For example,

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1 the major resonance forms for the conjugate base of acetone are CH2=C(CH3)–O– and 2 H2C––C(CH3)=O, with the former contributing more.3 Note 2: Resonance forms are connected by a double-headed arrow (↔). This must 4 not be confused with the double arrow connecting species in equilibrium (⇌).5 See also delocalization, Kekulé structure, resonance.67 resonance hybrid8 Molecular entity whose electronic structure is represented as the superposition of two 9 or more resonance forms (or Lewis structures) with different formal arrangements of

10 electrons but identical arrangements of nuclei.11 Note 1: Whereas each contributing resonance form represents a localized 12 arrangement of electrons which, considered by itself, would imply different bond lengths 13 (say) for formal single and double bonds, resonance between two or more contributors 14 requires each to have the same geometry, namely that of the resulting hybrid, which 15 represents a delocalized arrangement of electrons. A particular bond in the hybrid may 16 have a length that is, loosely, an “average” of formal single-bond and double-bond 17 values implied by its contributing individual resonance forms, but the hybrid does not 18 oscillate among these as if they were in equilibrium.19 Note 2: The resonance forms are connected by a double-headed arrow (↔), rather 20 than by the double arrow (⇄) representing equilibrium between species.2122 retrocycloaddition23 deprecated24 Cycloelimination.25 [3]2627 Ritchie equation28 Linear Gibbs-energy relation (linear free-energy relation)2930 lg {kN} = lg {k0} + N+

3132 applied to the reactions between nucleophiles and certain large and relatively stable 33 organic cations, e.g., arenediazonium, triarylmethyl, and aryltropylium cations, in 34 various solvents, where {kN} is the (reduced) second-order rate constant for reaction of 35 a given cation with a given nucleophilic system (i.e., given nucleophile in a given 36 solvent). {k0} is the (reduced) first-order rate constant for the same cation with water in 37 water, and N+ is a parameter characteristic of the nucleophilic system and independent 38 of the electrophilic reaction partner. 39 The discrepancy between second-order and first-order rate constants must be 40 reconciled by writing the equation with arguments of the logarithms of dimension 1, i.e.,

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1 by using reduced rate constants (as denoted by the curly brackets in the defining 2 equation):34 lg [kN/(dm3 mol-1s-1)] = lg (k0/s-1) + N+

56 Note 1: A surprising feature of the equation is the absence of a coefficient of N+ 7 characteristic of the substrate (cf. the s in the Swain-Scott equation), even though 8 values of N+ vary over 13 decadic log (lg) units. The equation thus involves a gigantic 9 breakdown of the reactivity-selectivity principle.

10 Note 2: The Ritchie equation is a special case of the more general Mayr-Patz 11 equation.12 See [193,372,373]. See also [125,374].13 rev[3]1415 -value (rho-value)16 Quantitative measure of the susceptibility of the rate constant or equilibrium constant 17 of an organic reaction to the influence of substituent groups, usually on an aromatic 18 ring.19 Note 1: Defined by Hammett to describe the effects of substituents at the meta- and 20 para-positions on rate or equilibrium of a reaction on the side chain of a substituted 21 benzene, the empirical equation has the general form2223 lg(kX/kH) or lg(KX/KH) = X

2425 in which X is a constant characteristic of the substituent X and of its position in the 26 reactant molecule.27 Note 2: More generally (not only for aromatic series), -values (modified with 28 appropriate subscripts and superscripts) are used to designate the susceptibility to 29 substituent effects of reactions of families of organic compounds, as given by the 30 modified set of -constants in an empirical correlation.31 Note 3: Reactions with a positive -value are accelerated (or the equilibrium 32 constants are increased) by substituents with positive -constants. Since the sign of 33 was defined so that substituents with a positive increase the acidity of benzoic acid, 34 such substituents are generally described as attracting electrons away from the 35 aromatic ring. It follows that reactions with a positive -value involve a transition state 36 (or reaction product) with an increased electron density at the reactive site of the 37 substrate.38 See also Hammett equation, -constant, Taft equation.39 rev[3]40

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1 -equation (rho-sigma equation)2 See Hammett equation, -value, -constant, Taft equation.3 [3]45 salt effect6 See kinetic electrolyte effect.7 [3]89 saturation transfer

10 See magnetization transfer.11 rev[3]1213 Saytzeff rule14 Preferential removal of a hydrogen from the carbon that has the fewest hydrogens in 15 dehydrohalogenation of secondary and tertiary haloalkanes.16 Note 1: The rule was originally formulated by A. Saytzeff (Zaitsev) to generalize the 17 orientation in -elimination reactions of haloalkanes. It has been extended and 18 modified, as follows: When two or more olefins can be produced in an elimination 19 reaction, the thermodynamically most stable alkene will predominate. 20 Note 2: Exceptions to the Saytzeff rule are exemplified by the Hofmann rule.21 See [375].22 See also Markovnikov rule.23 [3]2425 scavenger26 Substance that reacts with (or otherwise removes) a trace component (as in the 27 scavenging of trace metal ions) or traps a reactive intermediate.28 See also inhibition.29 [3]3031 selectivity32 Discrimination shown by a reagent in competitive attack on two or more substrates or 33 on two or more positions or diastereotopic or enantiotopic faces of the same substrate. 34 Note 1: Selectivity is quantitatively expressed by the ratio of rate constants of the 35 competing reactions, or by the decadic logarithm of such a ratio.36 Note 2: In the context of aromatic substitution (usually electrophilic, for 37 monosubstituted benzene derivatives), the selectivity factor Sf (expressing 38 discrimination between p- and m-positions in PhZ) is defined as 3940 Sf = lg (pf

Z/mfZ)

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12 where the partial rate factors pf

Z and mfZ express the reactivity of para and meta

3 positions in the aromatic compound PhZ relative to that of a single position in benzene.4 See [347].5 See also isoselective relationship, partial rate factor, regioselectivity, 6 stereoselectivity.7 rev[3]89 self-assembly

10 Process whereby a system of single-molecule components spontaneously forms an 11 organized structure, owing to molecular recognition.1213 shielding14 Extent to which the effective magnetic field is reduced for a nucleus in a molecule 15 immersed in an external magnetic field, relative to that experienced by a bare nucleus 16 in that field. 17 Note 1: The reduction is due to the circulation of the electrons around the observed 18 and the neighbouring nuclei. The external field induces a magnetic moment that is 19 oriented in the opposite direction to the external field, so that the local field at the central 20 nucleus is weakened, although it may be strengthened at other nuclei (deshielding).21 Note 2: This phenomenon is the origin of the structural dependence of the resonance 22 frequencies of the nuclei.23 See also chemical shift.24 [3]2526 shift reagent27 Paramagnetic substance that induces an additional change of the NMR resonance 28 frequency of a nucleus near any site in a molecule to which the substance binds. 29 See chemical shift.3031 sigma, pi32 See , 33 rev[3]3435 sigmatropic rearrangement36 Molecular rearrangement that involves both the creation of a new bond between 37 atoms previously not directly linked and the breaking of an existing bond. 38 Note 1: There is normally a concurrent relocation of bonds in the molecule 39 concerned, but neither the number of bonds nor the number of bonds changes. 40 Note 2: The transition state of such a reaction may be visualized as an association

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1 of two fragments connected at their termini by two partial bonds, one being broken 2 and the other being formed as, for example, the two allyl fragments in (a'). Considering 3 only atoms within the (real or hypothetical) cyclic array undergoing reorganization, if the 4 numbers of these in the two fragments are designated i and j, then the rearrangement

5 is said to be a sigmatropic change of order [i,j] (conventionally i ≤ j). Thus rearrangement 6 (a) is of order [3,3], whereas rearrangement (b) is a [1,5]sigmatropic shift of hydrogen. 7 (By convention the square brackets [...] here refer to numbers of atoms, in contrast with 8 current usage in the context of cycloaddition.)

91011 The descriptors a and s (antarafacial and suprafacial) may also be annexed to the 12 numbers i and j; (b) is then described as a [1s,5s] sigmatropic rearrangement, since it is 13 suprafacial with respect to both the hydrogen atom and the pentadienyl system:14

151617 See also cycloaddition, tautomerization.18 rev[3]1920 silylene21 (1) Generic name for H2Si: and substitution derivatives thereof, containing an 22 electrically neutral bivalent silicon atom with two non-bonding electrons. (The definition 23 is analogous to that given for carbene.)24 (2) The silanediyl group (H2Si<), analogous to the methylene group (H2C<).25 [3]2627 single-electron transfer mechanism (SET)28 Reaction mechanism characterized by the transfer of a single electron between two

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1 species, occurring in one of the steps of a multistep reaction.2 [3]34 single-step reaction 5 one-step reaction6 Reaction that proceeds through a single transition state (or no transition state).7 [3]89

10 skeletal formula11 bond-line formula12 Two-dimensional representation of a molecular entity in which bonds are indicated as 13 lines between vertices representing octet carbon atoms with attached hydrogens 14 omitted and in which other atoms are represented by their chemical symbols.15 Examples: ethanol, cyclohexene, 4-pyridone16

171819 See line formula.2021 Slater-type orbital (STO)22 Function centred on an atom for which the radial dependence has the form (r) ∝ rn–1

23 exp(–r), used to approximate atomic orbitals in the LCAO-MO method. 24 Note 1: n is the effective principal quantum number and is the orbital exponent 25 (screening constant) derived from empirical considerations. 26 Note 2: The angular dependence is usually introduced by multiplying the radial 27 function by a spherical harmonic Y1m(,). 28 Note 3: Owing to difficulties in computing the integrals of STOs analytically for 29 molecules with more than two atoms they are often replaced by linear combinations of 30 Gaussian orbitals.31 See [8].32 rev[3]3334 solvation

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1 Stabilizing interaction between a solute (or solute moiety) and the solvent. 2 Note: Such interactions generally involve electrostatic forces and van der Waals 3 forces, as well as chemically more specific effects such as hydrogen bond formation. 4 See also cybotactic region.5 [3]67 solvatochromic relationship8 Linear Gibbs-energy relationship (linear free-energy relationship) based on 9 solvatochromism.

10 See also Dimroth-Reichardt ET parameter, Kamlet-Taft solvent parameters.11 [3]1213 solvatochromism14 Pronounced change in position and sometimes intensity of an electronic absorption or 15 emission band, accompanying a change in the polarity of the medium.16 Note: Negative (positive) solvatochromism corresponds to a hypsochromic shift 17 (bathochromic shift) with increasing solvent polarity. 18 See [9,17,376].19 See also Dimroth-Reichardt ET parameter, Z-value.20 [3]2122 solvatomers23 Isomers that differ in their solvation environment.24 Note 1: Because the solvation environment fluctuates rapidly, solvatomers 25 interconvert rapidly.26 Note 2: Species that differ in the type of solvent molecules should not be called 27 solvatomers, because they are not isomers.2829 solvent parameter30 Quantity that expresses the capability of a solvent for interaction with solutes, based on 31 experimentally determined physicochemical quantities, in particular: relative 32 permittivity, refractive index, rate constants, Gibbs energies and enthalpies of reaction, 33 and ultraviolet-visible, infrared, and NMR spectra.34 Note 1: Solvent parameters are used in correlation analysis of solvent effects, either 35 in single-parameter or in multiple-parameter equations.36 Note 2: Solvent parameters include those representing a bulk property, such as 37 relative permittivity (dielectric constant) as well as those that describe a more localized 38 solute/solvent interaction, such as hydrogen-bonding acceptance or donation and 39 Lewis acid/base adduct formation. 40 See [17,377].

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1 See also: acceptor number, Catalán solvent parameters, Dimroth-Reichardt ET 2 parameter, Grunwald-Winstein equation, Kamlet-Taft solvent parameters, Laurence 3 solvent parameters, Koppel-Palm solvent parameters, linear solvation energy 4 relationship, Z-value.5 rev[3]67 solvolysis8 Reaction with solvent.9 Note 1: Such a reaction generally involves the rupture of one or more bonds in the

10 solute. More specifically the term is used for substitution, elimination, and fragmentation 11 reactions in which a solvent species serves as nucleophile or base. 12 Note 2: A solvolysis can also be classified as a hydrolysis, alcoholysis, or 13 ammonolysis, etc., if the solvent is water, alcohol, or ammonia, etc.14 Note 3: Often a solvolysis is a nucleophilic substitution (usually SN1, accompanied 15 by E1 elimination), where the nucleophile is a solvent molecule.16 rev[3]1718 SOMO19 Singly Occupied Molecular Orbital (such as the half-filled HOMO of a radical).20 See also frontier orbitals.21 [3]2223 special salt effect24 Steep increase of the rate of certain solvolysis reactions observed at low concentrations 25 of some non-common-ion salts.26 Note: The effect is attributed to trapping of an intimate ion pair that would revert to 27 reactant in the absence of the salt.28 See also kinetic electrolyte effect.29 rev[3]3031 specific catalysis32 Acceleration of a reaction by a unique catalyst, rather than by a family of related 33 substances. 34 Note: The term is most commonly used in connection with specific hydrogen-ion or 35 hydroxide-ion (lyonium ion or lyate ion) catalysis. 36 See also general acid catalysis, general base catalysis, pseudo-catalysis.37 [3]3839 spectator mechanism40 Pre-association mechanism in which one of the molecular entities, C, is already present

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1 in an encounter pair with A during formation of B from A, but does not assist the 2 formation of B, e.g.,3

456 Note: The formation of B from A may itself be a bimolecular reaction with some other 7 reagent. Since C does not assist the formation of B, it is described as being present as 8 a spectator.9 See also microscopic diffusion control.

10 [3]1112 spin adduct13 See spin trapping.14 [3]1516 spin counting17 See spin trapping.18 [3]1920 spin density21 Unpaired electron density at a position of interest, usually at carbon, in a radical or a 22 triplet state.23 Note: Spin density is often measured experimentally by electron paramagnetic 24 resonance/electron spin resonance (EPR/ ESR) spectroscopy through hyperfine 25 splitting of the signal by neighbouring magnetic nuclei.26 See also radical centre.27 [3]2829 spin label30 Stable paramagnetic group (typically an aminoxy radical, R2NO) that is attached to a 31 part of a molecular entity whose chemical environment may be revealed by its electron 32 spin resonance (ESR) spectrum.33 Note: When a paramagnetic molecular entity is used without covalent attachment to 34 the molecular entity of interest, it is frequently referred to as a spin probe.35 [3]3637 spin trapping38 Formation of a more persistent radical from interaction of a transient radical with a

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1 diamagnetic reagent.2 Note 1: The product radical accumulates to a concentration where detection and, 3 frequently, identification are possible by EPR/ESR spectroscopy. 4 Note 2: The key reaction is usually one of attachment; the diamagnetic reagent is 5 said to be a spin trap, and the persistent product radical is then the spin adduct. The 6 procedure is referred to as spin trapping, and is used for monitoring reactions involving 7 the intermediacy of reactive radicals at concentrations too low for direct observation. 8 Typical spin traps are C-nitroso compounds and nitrones, to which reactive radicals will 9 rapidly add to form aminoxy radicals.

10 Example:

1112 Note 3: A quantitative development in which essentially all reactive radicals 13 generated in a particular system are intercepted has been referred to as spin counting.14 Note 4: Spin trapping has also been adapted to the interception of radicals generated 15 in both gaseous and solid phases. In these cases the spin adduct is in practice 16 transferred to a liquid solution for EPR/ESR observation.17 [3]1819 stable20 Having a lower standard Gibbs energy, compared to a reference chemical species.21 Note 1: Quantitatively, in terms of Gibbs energy, a chemical species A is more stable 22 than its isomer B if ∆rG° is positive for the (real or hypothetical) reaction A → B. 23 Note 2: For the two reactions2425 P → X + Y ∆rG1°26 and Q → X + Z ∆rG2°2728 if ∆rG1° > ∆rG2°, then P is more stable relative to its product Y than is Q relative to Z.29 Note 3: Both in qualitative and quantitative usage the term stable is therefore always 30 used in reference to some explicitly stated or implicitly assumed standard.31 Note 4: The term should not be used as a synonym for unreactive or less reactive 32 since this confuses thermodynamics and kinetics. A relatively more stable chemical 33 species may be more reactive than some reference species towards a given reaction 34 partner.35 See also inert, unstable.36 [3]

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12 stationary state3 (1) (in quantum mechanics): Wavefunction whose probability density | |2 remains 4 constant and whose observable properties do not evolve with time.5 (2) (in kinetics): See steady state.6 [3]78 steady state (or stationary state)9 (1) Approximation that the kinetic analysis of a complex reaction involving unstable

10 intermediates in low concentration can be simplified by setting the rate of change of 11 each such intermediate equal to zero, so that the rate equation can be expressed as a 12 function of the concentrations of chemical species present in macroscopic amounts.13 For example, if X is an unstable intermediate in the reaction sequence:

141516 Since [X] is negligibly small, d[X]/dt, the rate of change of [X], can be set equal to zero. 17 The steady state approximation then permits solving the following equation1819 d[X]/dt = k1[A] – k–1[X] – k2[X][C] = 02021 to obtain the steady-state [X]:2223 [X] = k1[A]/(k–1 + k2[C])2425 whereupon the rate of reaction is expressed:2627 d[D]/dt = k2[X][C] = k1k2[A][C]/(k–1 + k2[C])2829 Note: The steady-state approximation does not imply that [X] is even approximately 30 constant, only that its absolute rate of change is very much smaller than that of [A] and 31 [D].32 (2) Regime in a stirred flow reactor such that all concentrations are independent of time.33 See [13].34 [3]3536 stepwise reaction37 Chemical reaction with at least one reaction intermediate and involving at least two 38 consecutive elementary reactions.

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1 See also composite reaction, reaction step.2 [3]34 stereoelectronic5 Pertaining to the dependence of the properties (especially energy or reactivity) of a 6 molecular entity or of a transition state on the relative disposition of electron pairs owing 7 to the nuclear geometry. 8 Note: Stereoelectronic effects are ascribed to the differing overlaps of atomic orbitals 9 in different conformations.

10 See [11].11 [3]1213 stereogenic centre14 Atom within a molecule bearing groups such that interchanging any two of them leads 15 to a stereoisomer of the original molecule. 16 See [11,378].1718 stereoisomers19 Isomers that have the same bonds (connectivity) but differ in the arrangement of their 20 atoms and cannot be interconverted by rapid rotation around single bonds.21 See [11].2223 stereoselectivity (stereoselective)24 Preferential formation in a chemical reaction of one stereoisomer over another.25 Note 1: When the stereoisomers are enantiomers, the phenomenon is called 26 enantioselectivity and is quantitatively expressed by the enantiomeric excess or 27 enantiomeric ratio; when they are diastereoisomers, it is called diastereoselectivity and 28 is quantitatively expressed by the diastereomeric excess or diastereomeric ratio. 29 Note 2: Reactions are termed 100 % stereoselective if the preference is complete, 30 or partially (x %) stereoselective if one product predominates. The preference may also 31 be referred to semiquantitatively as high or low stereoselectivity32 See [11,140].33 [3]3435 stereospecificity (stereospecific)36 Property of those chemical reactions in which different stereoisomeric reactants are 37 converted into different stereoisomeric products.38 Example: electrocyclization of trans,cis,trans-octa-2,4,6-triene produces cis-5,6-39 dimethylcyclohexa-1,3-diene, whereas cis,cis,trans-octa-2,4,6-triene produces racemic 40 trans-5,6-dimethylcyclohexa-1,3-diene.

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1

234 Note 1: A stereospecific process is necessarily stereoselective but not all 5 stereoselective processes are stereospecific. Stereospecificity may be total (100 %) or 6 partial.7 Note 2: The term is also applied to situations where reaction can be performed with 8 only one stereoisomer. For example the exclusive formation of racemic trans-1,2-9 dibromocyclohexane upon bromination of cyclohexene is a stereospecific process,

10 even though the analogous reaction with (E)-cyclohexene has not been performed.11 Note 3: Stereospecificity does NOT mean very high stereoselectivity. This usage is 12 unnecessary and is strongly discouraged.13 See [11,140].14 For the term stereospecific polymerization see [379].15 rev[3]1617 steric-approach control18 Situation in which the stereoselectivity of a reaction under kinetic control is governed 19 by steric hindrance to attack of the reagent, which is directed to the less hindered face 20 of the molecule.21 Note: Partial bond making at the transition state must be strong enough for steric 22 control to take place, but the transition state should not be so close to products that the 23 steric demand of the reagent at the transition state is the same as the steric demand of 24 the group as present in the product.25 An example is LiAlH4 reduction of 3,3,5-trimethylcyclohexan-1-one, where steric 26 hindrance by an axial methyl directs hydride addition to the equatorial position, even 27 though the more stable product has the H axial and the OH equatorial.28 See also product-development control.29 rev[3]3031

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12 steric effect3 Consequences for molecular geometry, thermochemical properties, spectral features, 4 solvation, or reaction rates resulting from the fact that atoms repel each other at close 5 distance. The repulsion is due to the quantum-mechanical Pauli exclusion principle. 6 Substitution of hydrogen atoms by groups with a larger van der Waals radius may lead 7 to situations where atoms or groups of atoms repel each other, thereby affecting 8 distances and angles.9 Note 1: It is in principle difficult to separate the steric effect from other electronic

10 effects.11 Note 2: For the purpose of correlation analysis or linear Gibbs-energy relations 12 (linear free-energy relations) various scales of steric parameters have been proposed, 13 notably A values, Taft's ES and Charton's v scales.14 Note 3: A steric effect on a rate process may result in a rate increase (steric 15 acceleration) or a decrease (steric retardation) depending on whether the transition 16 state or the reactant state is more affected by the steric effect.17 Note 4: Bulky groups may also attract each other if at a suitable distance.18 See [290,380]. 19 See Taft equation, van der Waals forces.20 rev[3]2122 steric hindrance23 Steric effect whereby the crowding of substituents around a reaction centre retards the 24 attack of a reagent.25 rev[3]2627 stopped flow28 Technique for following the kinetics of reactions in solution (usually in the millisecond 29 time range) in which two, or more, reactant solutions are rapidly mixed by being forced 30 through a mixing chamber. The flow of the mixed solution along a uniform tube is then 31 suddenly arrested. At a fixed position along the tube the solution is monitored as a 32 function of time following the stoppage of the flow by a method with a rapid response 33 (e.g., optical absorption spectroscopy).34 See mixing control.35 rev[3]3637 strain38 Feature of a molecular entity or transition structure for which the energy is increased 39 because of unfavourable non-bonded (steric) interactions, bond lengths, bond angles, 40 or dihedral angles (torsional strain), relative to a standard.

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1 Note 1: The strain energy is quantitatively defined as the standard enthalpy of a 2 structure relative to that of a strainless structure (real or hypothetical) made up from the 3 same atoms with the same types of bonding. 4 For example, the enthalpy of formation of cyclopropane is +53.6 kJ mol-1, whereas 5 the hypothetical enthalpy of formation based on three "normal" methylene groups, from 6 acyclic models, is -62 kJ mol-1. On this basis cyclopropane is destabilized by ca. 115 kJ 7 mol–1 of strain energy. 8 See molecular mechanics.9 [3]

1011 structural isomers12 discouraged term for constitutional isomers.1314 subjacent orbital15 Next-to-Highest Occupied Molecular Orbital (NHOMO, also called HOMO–1). 16 Note: Subjacent and superjacent orbitals sometimes play an important role in the 17 interpretation of molecular interactions according to the frontier orbital approach. 18 See [381].19 [3]2021 substituent22 Any atom or group of bonded atoms that can be considered to have replaced a 23 hydrogen atom (or two hydrogen atoms in the special case of bivalent groups) in a 24 parent molecular entity (real or hypothetical).25 [3]2627 substitution28 Chemical reaction, elementary or stepwise, of the form A–B + C → A–C + B, in which 29 one atom or group in a molecular entity is replaced by another atom or group. 30 Examples31 CH3Cl + HO– → CH3OH + Cl–

32 C6H6 + NO2+ → C6H5NO2 + H+

33 Note: A substitution reaction can be distinguished as an electrophilic substitution or 34 a nucleophilic substitution, depending on the nature of the reactant that is considered 35 to react with the substrate.36 rev[3]3738 substrate39 Chemical species, the reaction of which with some other chemical reagent is under 40 observation (e.g., a compound that is transformed under the influence of a catalyst).

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1 Note: The term should be used with care. Either the context or a specific statement 2 should always make it clear which chemical species in a reaction is regarded as the 3 substrate.4 See also transformation.5 [3]67 successor complex8 Chemical species formed by the transfer of an electron or of a hydrogen (atom or ion) 9 from a donor D to an acceptor A after these species have diffused together to form the

10 precursor or encounter complex:

1112 rev[3]1314 suicide inhibition15 See mechanism-based inhibition.16 [3]1718 superacid19 Medium having a high acidity, generally greater than that of 100 % sulfuric acid. The 20 common superacids are made by dissolving a powerful Lewis acid (e.g., SbF5) in a 21 suitable Brønsted acid, such as HF or HSO3F.22 Note 1: An equimolar mixture of HSO3F and SbF5 is known by the trade name Magic 23 Acid.24 Note 2: An uncharged gas-phase substance having an endothermicity (enthalpy) of 25 deprotonation (dehydronation) lower than that of H2SO4 is also called a superacid. 26 Nevertheless such a superacid is much less acidic in the gas phase than similar cationic 27 acids.28 See [18,382,383,384].29 See acidity, superbase.30 rev[3]3132 superbase33 Compound having a very high basicity.34 Examples include amide bases such as LDA, potassium tert-butoxide + 35 organolithium, some phosphazenes.36 See superacid.37 See [385].38 rev[3]39

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1 superjacent orbital2 Second Lowest Unoccupied Molecular Orbital (SLUMO). 3 Note: Subjacent and superjacent orbitals sometimes play an important role in the 4 interpretation of molecular interactions according to the frontier orbital approach. 5 See [381].67 suprafacial8 See antarafacial.9 [3]

1011 supramolecular12 Description of a system of two or more molecular entities that are held together and 13 organized by means of intermolecular (noncovalent) binding interactions.14 See [386].15 [3]1617 Swain-Lupton equation18 Dual-parameter approach to the correlation analysis of substituent effects, which 19 involves a field constant (F) and a resonance constant (R).2021 lg(kX/kH) = fFX + rRX

2223 See [191,387].24 Note 1: The original treatment was modified later.25 Note 2: The procedure has often been applied, but also often criticized.26 See [388,389,390,391,392].27 [3]2829 Swain-Scott equation30 Linear Gibbs-energy relation (linear free-energy relation) of the form3132 lg(k/k0) = sn3334 applied to the variation of reactivity of a given electrophilic substrate towards a series 35 of nucleophilic reagents, where k0 is a rate constant for reaction with water, k is the 36 corresponding rate constant for reaction with any other nucleophilic reagent, n is a 37 measure of the nucleophilicity of the reagent (n = 0.0 for water) and s is a measure of 38 the sensitivity of the substrate to the nucleophilicity of the reagent (s = 1.0 for CH3Br). 39 See [192].40 See also Mayr-Patz equation, Ritchie equation.

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1 [3]23 symproportionation4 comproportionation5 [3]67 syn8 See anti.9 See [11].

10 [3]1112 synartetic acceleration13 See neighbouring group participation.14 [3]1516 synchronization17 See principle of nonperfect synchronization.18 rev[3]1920 synchronous21 Feature of a concerted process in which all the changes (generally bond rupture and 22 bond formation) have progressed to the same extent at the transition state. 23 Note 1: A synchronous reaction is distinguished from (1) a concerted reaction, which 24 takes place in a single kinetic step without being synchronous, (2) a reaction where 25 some of the changes in bonding take place earlier, followed by the rest, and (3) a two-26 step reaction, which takes place in two kinetically distinct steps, via a stable 27 intermediate 28 Note 2: The progress of the bonding changes (or other primitive changes) is not 29 defined quantitatively in terms of a single parameter applicable to different bonds. The 30 concept therefore does not admit an exact definition except in the case of concerted 31 processes involving changes in two identical bonds. If the bonds are not identical, the 32 process should simply be described as concerted.33 See [393,394,395]. For an index of synchronicity see [396].34 See also imbalance.35 rev[3]3637 , 38 Symmetry designations that distinguish molecular orbitals as being symmetric () or 39 antisymmetric () with respect to a defining plane containing at least one atom. 40 Note 1: In practice the terms are used both in this rigorous sense (for orbitals

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1 encompassing the entire molecule) and for localized two-centre orbitals or bonds. In 2 the case of localized two-centre bonds, a bond has a nodal plane that includes the 3 internuclear bond axis, whereas a bond has no such nodal plane. (A bond in 4 organometallic or inorganic chemical species has two nodes.) Radicals are classified 5 by analogy into and radicals.6 Note 2: Such two-centre orbitals may take part in molecular orbitals of or 7 symmetry. For example, the methyl group in propene contains three C–H bonds, each 8 of which is of local symmetry (i.e., without a nodal plane including the internuclear 9 axis), but these three bonds can in turn be combined to form a set of group orbitals

10 one of which has symmetry with respect to the principal molecular plane and can 11 accordingly interact with the two-centre orbital of symmetry ( bond) of the double-12 bonded carbon atoms, to form a molecular orbital of symmetry. Such an interaction 13 between the CH3 group and the double bond is an example of hyperconjugation. This 14 cannot rigorously be described as - conjugation since and here refer to different 15 defining planes, and interaction between orbitals of different symmetries (with respect 16 to the same defining plane) is forbidden.17 Note 3: Conjugation between a system and a lone pair of (for example) an ether 18 oxygen involves a lone pair of symmetry with respect to the defining plane. It is 19 incorrect to consider this as an interaction between the system and one of two 20 identical sp3-hybrid lone pairs, which are neither nor .21 Note 4: The two lone pairs on a carbonyl oxygen are properly classified as or 22 with respect to the plane perpendicular to the molecular plane (dotted line perpendicular 23 to the plane of the page), rather than as two sp2-hybridized lone pairs. This distinction 24 readily accounts for the facts that there are two different lone-pair ionization energies 25 and two different n-* excited states.26

272829303132 See also [397].3334 -adduct35 Product formed by the attachment of an electrophilic or nucleophilic entering group or 36 of a radical to a ring carbon of an aromatic species so that a new bond is formed and 37 the original conjugation is disrupted.38 Note 1: This has generally been called a complex, or a Wheland complex from

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1 electrophilic addition, but adduct is more appropriate.2 Note 2: The term may also be used for analogous adducts to systems.3 See also Meisenheimer adduct.4 rev[3]56 -constant 7 Substituent constant for meta- and para-substituents in benzene derivatives as defined 8 by Hammett on the basis of the ionization constant of a substituted benzoic acid, i.e., 9 lg(Ka

X/KaH), where Ka

X is the ionization constant (acid-dissociation constant) of a m- or 10 p-substituted benzoic acid and Ka

H that of benzoic acid itself.11 Note 1: A large positive -value implies high electron-withdrawing power by an 12 inductive and/or resonance effect, relative to H; a large negative -value implies high 13 electron-releasing power relative to H.14 Note 2: The term is also used as a collective description for related electronic 15 substituent constants based on other standard reaction series, of which, +,– and o 16 are typical; also for constants which represent dissected electronic effects, such as I 17 and R. For example,– (sigma-minus) constants are defined on the basis of the 18 ionization constants of para-substituted phenols, where such substituents as nitro show 19 enhanced electron-withdrawing power.20 See [124,125,126,191,226,398]. 21 See also Hammett equation, -value, Taft equation.22 [3]2324 Taft equation25 Linear free-energy relation (linear Gibbs-energy relation) involving the polar substituent 26 constant * and the steric substituent constant Es, as derived from reactivities of 27 aliphatic esters2829 lg {k} = lg {k0} + ρ*σ* + Es

3031 The argument of the lg function should be of dimension 1. Thus, the reduced rate 32 constants should be used, i.e., the rate coefficient divided by its units: {k} = k/[k] and 33 {k0} = k0/[k0].34 Note: The standard reaction (k0) is the hydrolysis of methyl acetate, whereby Es is 35 evaluated from the rate of acid-catalyzed hydrolysis, relative to that of methyl acetate, 36 and * is evaluated from the ratio of the rates of base- and acid-catalyzed hydrolysis.37 See [172,380,399,400]. 38 See also Hammett equation, -value, -constant.39 rev[3]40

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1 tautomerization2 Rapid isomerization of the general form3 G–X–Y=Z ⇌ X=Y–Z–G4 where the isomers (called tautomers) are readily interconvertible.5 Note 1: The atoms of the groups X,Y, Z are typically any of C, H, N, O, or S, and G 6 is a group that becomes an electrofuge or nucleofuge during isomerization.7 Note 2: The commonest case, when the electrofuge is H+, is also known as a 8 prototropic rearrangement.9 Examples:

10

111213 Note 3: The group Y may itself be a three-atom (or five-atom) chain extending the 14 conjugation, as in15

161718 Note 4: Ring-chain tautomerization is the case where addition across a double bond 19 leads to ring formation, as in20

2122 Note 5: Valence tautomerization is the case of rapid isomerization involving the 23 formation and rupture of single and/or double bonds, without migration of atoms: for 24 example

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12 See [346,401].3 See also ambident, fluxional, sigmatropic rearrangement, valence tautomerization.4 rev[3]56 tautomers7 Constitutional isomers that can interconvert more or less rapidly, often by migration of 8 a hydron.9 See tautomerization.

1011 tele-substitution12 Substitution reaction in which the entering group takes up a position more than one 13 atom away from the atom to which the leaving group was attached.14 Example15

1617 See also cine-substitution.18 See [402,403].19 rev[3]2021 termination22 Step(s) in a chain reaction in which reactive intermediates are destroyed or rendered 23 inactive, thus ending the chain.24 [3]2526 tetrahedral intermediate27 Reaction intermediate in which the bond arrangement around an initially double-bonded 28 carbon atom (typically a carbonyl carbon) has been transformed from tricoordinate to 29 tetracoordinate (with a coordination number of 4).30 Example:

3132 rev[3]3334

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1 thermodynamic control (of product composition)2 equilibrium control3 Conditions (including reaction times) that lead to reaction products in a proportion 4 specified by the equilibrium constant for their interconversion.5 See also kinetic control.6 rev[3]78 thermolysis9 Uncatalysed cleavage of one or more covalent bonds resulting from exposure of a

10 molecular entity to a raised temperature, or a process of which such cleavage is an 11 essential part.12 See also pyrolysis.13 [3]1415 through-conjugation16 Phenomenon whereby electrons can be delocalized from any of three (or more) groups 17 to any other.18 Example: p-XC6H4Y, where an electron pair can be delocalized from electron-19 donating X not only to ring carbons but also to electron-withdrawing Y. 20 Note 1: This may be contrasted with cross-conjugation.21 Note 2: In Hammett-type correlations (linear Gibbs-energy relationships) this 22 situation can lead to exalted substituent constants + or –, as in solvolysis of p-23 CH3OC6H4C(CH3)2Cl or acidity of p-nitrophenol, respectively. 2425 TICT26 See twisted intramolecular charge transfer.2728 torquoselectivity29 Preference for inward or outward rotation of substituents in conrotatory or disrotatory 30 electrocyclic ring-opening and ring-closing reactions, often owing to a preference for 31 electron donors (especially including large groups) to rotate outward and acceptors to 32 rotate inward.33 See [404,405].

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12 rev[3]34 transferability5 Assumption that a chemical property associated with an atom or group of atoms in a 6 molecule will have a similar (but not identical) value in other circumstances.7 Examples: equilibrium bond length, bond force constant, NMR chemical shift.8 [3]9

10 transformation11 Conversion of a substrate into a particular product, irrespective of the specific reagents 12 or mechanisms involved.13 Example: transformation of aniline (C6H5NH2) into N-phenylacetamide 14 (C6H5NHCOCH3), which may be effected with acetyl chloride or acetic anhydride or 15 ketene.16 Note: A transformation is distinct from a reaction, the full description of which would 17 state or imply all the reactants and all the products.18 See [406].19 [3]2021 transient (chemical) species22 Short-lived reaction intermediate.23 Note 1: Transiency can be defined only in relation to a time scale fixed by the 24 experimental conditions and by the limitations of the technique employed in the 25 detection of the intermediate. The term is a relative one.

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1 Note 2: Transient species are sometimes also said to be metastable. However, this 2 latter term should be avoided, because it relates a thermodynamic term to a kinetic 3 property, although most transients are also thermodynamically unstable with respect to 4 reactants and products.5 See also persistent.6 [3]78 transition dipole moment9 Vector quantity describing the oscillating electronic moment induced by an

10 electromagnetic wave, given by an integral involving the dipole moment operator m and 11 the ground- and excited-state wavefunctions:1213 M = exc m gnd d1415 Note: The magnitude of this quantity describes the allowedness of an electronic 16 transition. It can often be separated into an electronic factor and a nuclear-overlap factor 17 known as the Franck-Condon factor.18 See [9].1920 transition state21 State of a molecular system from which there are equal probabilities of evolving toward 22 states of lower energy, generally considered as reactants and products of an 23 elementary reaction.24 Note 1: The transition state corresponds to the maximum along the minimum-Gibbs-25 energy path connecting reactants and products.26 Note 2: The transition state can be considered to be a chemical species of transient 27 existence.28 Note 3: The term transition-state structure refers to a structure inferred from kinetic 29 and stereochemical investigations; it does not necessarily coincide with a transition 30 state or with a transition structure, which corresponds to a saddle point on a potential-31 energy surface, although it may represent an average structure.32 Note 4: The assembly of atoms at the transition state has also been called an 33 activated complex, although it is not a complex according to the definition in this 34 Glossary.35 Note 5: There are also reactions, such as the gas-phase colligation of simple radicals 36 or the reactions of some reactive intermediates in solution, that do not require activation 37 and do not involve a transition state.38 Note 6: Ultrafast spectroscopy permits observation of transition states in some 39 special cases.40 See [349,407].

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1 See also Gibbs energy of activation, potential-energy profile, reaction coordinate, 2 transition structure.3 rev[3]45 transition-state analogue6 Species designed to mimic the geometry and electron density of the transition state of 7 a reaction, usually enzymatic.8 Note: A transition-state analogue is usually not a substrate for the enzyme, but rather 9 an inhibitor.

10 rev[3]1112 transition structure 13 Molecular entity corresponding to a saddle point on a potential-energy surface, with one 14 negative force constant and its associated imaginary frequency.15 Note 1: Whereas the transition state is not a specific molecular structure, but a set 16 of structures between reactants and products, a transition structure is one member of 17 that set with a specific geometry and energy.18 Note 2: Although the saddle point coincides with the potential-energy maximum 19 along a minimum-energy reaction path, it does not necessarily coincide with the 20 maximum of Gibbs energy for an ensemble of chemical species.21 Note 3: The term transition-state structure is not a synonym for transition structure.22 See [349,408].23 See also activated complex, transition state.24 rev[3]2526 transition vector27 Normal mode of vibration of a transition structure corresponding to the single imaginary 28 frequency and tangent to the intrinsic reaction coordinate at the saddle point. 29 Sometimes called the reaction-coordinate vibrational mode.30 Note: Infinitesimal motion along the transition vector in the two opposite senses 31 determines the initial direction leading either toward reactants or toward products.32 Note 2: The term "transition coordinate" was used in the 1994 Glossary of Terms in 33 Physical Organic Chemistry in the sense defined here for transition vector, but 34 apparently this usage was unique in the literature at that time and has not been 35 generally adopted since.36 See [349,409].37 See reaction coordinate, transition state.383940

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1 transport control2 See encounter-controlled rate, microscopic diffusion control.3 [3]45 trapping6 Interception of a reactive molecule or reaction intermediate so that it is removed from 7 the system or converted into a more stable form for study or identification.8 See also scavenger.9 [3]

1011 triplet state12 State having a total electron spin quantum number of 1.13 See [9]. 1415 tunnelling16 Quantum-mechanical phenomenon by which a particle or a set of particles penetrates 17 a barrier on its potential-energy surface without having the energy required to surmount 18 that barrier.19 Note 1: In consequence of Heisenberg's Uncertainty Principle, a molecular entity has 20 a nonzero probability of adopting a geometry that is classically forbidden because it 21 corresponds to a potential energy greater than the total energy.22 Note 2: Tunnelling is often considered to be a correction to an (over)simplified 23 version of transition-state theory.24 Note 3: Because the rate of tunnelling increases with decreasing mass, it is 25 significant in the context of isotope effects, especially of hydrogen isotopes.26 See [410,411].27 rev[3]2829 twisted intramolecular charge transfer (TICT)30 Feature of an excited electronic state formed by intramolecular electron transfer from 31 an electron donor (D) to an electron acceptor (A), where interaction between electron 32 and hole is restricted because D+ and A– are perpendicular to each other. 33 See [412].3435 umpolung36 Process by which the nucleophilic or electrophilic property of a functional group is 37 reversed.38 Note: Umpolung is often achieved by temporary exchange of heteroatoms N or O by 39 others, such as P, S, or Se, as in the conversion of electrophilic RCH=O to RCH(SR')2

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1 and then with base to nucleophilic RC(SR')2–. Also the transformation of a haloalkane

2 RX into a Grignard reagent RMgCl is an umpolung.3 See [413].4 rev[3]56 unimolecular7 Feature of a reaction in which only one molecular entity is involved.8 See molecularity.9 rev[3]

1011 unreactive12 Failing to react with a specified chemical species under specified conditions.13 Note: The term should not be used in place of stable, which refers to a 14 thermodynamic property, since a relatively more stable species may nevertheless be 15 more reactive than some reference species towards a given reaction partner.16 [3]1718 unstable19 Opposite of stable, i.e., the chemical species concerned has a higher molar Gibbs 20 energy than some assumed standard.21 Note: The term should not be used in place of reactive or transient, although more 22 reactive or transient species are frequently also more unstable.23 rev[3]2425 upfield26 superseded but still widely used to mean shielded.27 See chemical shift.28 [3]2930 valence31 Maximum number of single bonds that can be commonly formed by an atom or ion of 32 the element under consideration.33 Note: Often there is a most common maximum for a given element, and atoms in 34 compounds where this number is exceeded, such as pentacoordinate carbocations 35 (“carbonium ions”) and iodine(III) compounds, are called hypervalent.36 rev[3]3738 valence isomer39 Constitutional isomer related to another by pericyclic reaction.

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1 Examples: Dewar benzene, prismane, and benzvalene are valence isomers of 2 benzene.3 Note: Valence isomers are separable, as distinguished from valence tautomers, 4 which interconvert rapidly. See valence tautomerization.5 rev[3]67 valence tautomerization8 Rapid isomerization involving the formation and rupture of single and/or double bonds, 9 without migration of atoms.

10 Example:

1112 See tautomerization.13 rev[3]1415 van der Waals forces16 Attractive or repulsive forces between molecular entities (or between groups within the 17 same molecular entity) other than those due to bond formation or to the electrostatic 18 interaction of ions or of ionic groups with one another or with neutral molecules.19 Note 1: The term includes dipole-dipole, dipole-induced dipole, and London 20 (instantaneous induced dipole-induced dipole) forces, as well as quadrupolar forces.21 Note 2: The term is sometimes used loosely for the totality of nonspecific attractive 22 or repulsive intermolecular forces.23 Note 3: In the context of molecular mechanics, van der Waals forces correspond only 24 to the London dispersion forces plus the Pauli repulsion forces. Interactions due to the 25 average charge distribution (and not to fluctuations around the average or to induced 26 dipoles), including dipole-dipole, ion-ion and ion-dipole forces, are called Coulombic 27 forces.28 See [290,414].29 See also dipole-dipole interaction, dipole-induced dipole forces, London forces.30 rev[3]3132 volume of activation, ∆‡V33 Quantity derived from the pressure dependence of the rate constant of a reaction, 34 defined by the equation3536 ∆‡V = RT T[∂ln(𝑘/[𝑘])/∂𝑝]37

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1 provided that rate constants of all reactions (except first-order reactions) are expressed 2 in pressure-independent concentration units, such as mol dm–3 at a fixed temperature 3 and pressure. The argument in the lg function should be of dimension 1. Thus, the rate 4 constant should be divided by its units, [k].5 Note: The volume of activation is interpreted as the difference between the partial 6 molar volume ‡V of the transition state and the sums of the partial molar volumes of the 7 reactants at the same temperature and pressure, i.e.,89 ∆‡V = ‡V – (VR)

1011 where is the order in the reactant R and VR its partial molar volume.12 See [13].13 See order of reaction.14 rev[3]1516 water/octanol partition coefficient (partition ratio)17 See octanol-water partition ratio1819 wavefunction 20 A mathematical expression whose form resembles the wave equations of physics, 21 supposed to contain all the information associated with a particular atomic or molecular 22 system. In particular, a solution of the Schrödinger wave equation, H = E, as an 23 eigenfunction of the hamiltonian operator H, which involves the electronic and/or 24 nuclear coordinates.25 Note 1: The wavefunction contains all the information describing an atomic or 26 molecular system that is consistent with the Heisenberg Uncertainty Principle.27 Note 2: When a wavefunction is operated on by certain quantum-mechanical 28 operators, a theoretical evaluation of physical and chemical observables for that system 29 (the most important one being energy) can be carried out.30 See [8,12]. 3132 Wheland intermediate33 See Meisenheimer adduct, -adduct.34 [3]3536 Woodward-Hoffmann rules37 See orbital symmetry.383940

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1 ylide2 Chemical species that can be produced by loss of a hydron from an atom directly 3 attached to the central atom of an onium ion.4 Example:5 Ph3P+–CHRR’ → [Ph3P+–C–RR’ ↔ Ph3P=CRR’] + H+

6 See [40].7 [3]89 Yukawa-Tsuno equation

10 Multiparameter extension of the Hammett equation to quantify the role of enhanced 11 resonance effects on the reactivity of para–substituted benzene derivatives, 1213 lg {k} = lg {ko} + [o + r(+ –o)]14 or lg {k} = lg {ko} + [σo + r(– –o)]1516 where the parameter r expresses the enhancement and where o is a substituent 17 constant based on reactivities of phenylacetic acids and similar substrates, where 18 resonance interaction is weak or absent. The argument in the lg function should be of 19 dimension 1. Thus, reduced rate constants should be used {k}= k/[k] and {ko} = ko/[ko].20 See [398,415,416].21 See also dual substituent-parameter equation, -value, -constant, through-22 conjugation.23 rev[3]2425 Zaitsev rule26 See Saytzeff rule.27 [3]2829 zero-point energy30 Extent, in consequence of Heisenberg's Uncertainty Principle, by which a particle or a 31 set of particles has an energy greater than that of the minimum on the potential-energy 32 surface.33 Note 1: Because of zero-point energy a molecular entity has a nonzero probability of 34 adopting a geometry whose energy is greater than that of the energy minimum. 35 Note 2: A molecular entity with zero-point energy may even adopt a geometry with 36 a potential energy greater than its total energy, a possibility that permits tunnelling.37 Note 3: Because the magnitude of zero-point energy increases with decreasing 38 mass, it is significant in the context of isotope effects, especially of hydrogen isotopes.3940 Zucker-Hammett hypothesis

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1 Assumption that if lg {k1} (= k1/[k1], reduced pseudo-first-order rate constant of an acid-2 catalyzed reaction) is linear in Ho (Hammett acidity function), then water is not involved 3 in the transition state of the rate-controlling step, whereas if lg {k1} is linear in lg {[H+]} 4 then water is involved. The argument in the lg function should be of dimension 1. Thus, 5 reduced concentration = {[H+]} should be used, i.e., concentration of protons divided by 6 its units.7 Note: This has been shown to be an overinterpretation.8 See [22,417].9 See also Bunnett-Olsen equation, Cox-Yates equation.

10 rev[3]1112 Z-value13 Quantitative measure of solvent polarity based on the UV-vis spectrum of 1-ethyl-4-14 (methoxycarbonyl)pyridinium iodide.15 See [143].16 See solvent parameter.17 rev[3]1819 zwitterion20 Highly dipolar, net uncharged (neutral) molecule having full electrical charges of 21 opposite sign, which may be delocalized within parts of the molecule but for which no 22 uncharged canonical resonance structure can be written.

23 Examples: glycine (H3N+–CH2–CO2−), betaine (Me3N+–CH2–CO2

−).24 Note 1: Sometimes also referred to as inner salts or ampholytes.25 Note 2: Mesoionic compounds, such as sydnones, in which both positive and 26 negative charge are delocalized, are sometimes considered as zwitterions, but species 27 with a localized nonzero formal charge, such as a nitrone, CH3CH=N+(–O–)CH3, are 28 not.29 See [418].30 rev[3]31

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380 M. Charton, Prog. Phys. Org. Chem. 16, 287 (1987).

381 J. A. Berson, Acc. Chem. Res. 5, 406 (1972).

382 R. J. Gillespie, Acc. Chem. Res. 1, 202 (1968).

383 G. A. Olah, A. T. Ku, J. A. Olah, J. Org. Chem. 35, 3904 (1970).

384 I. A. Koppel, P. Burk, I. Koppel, I. Leito, T. Sonoda, M. Mishima, J. Am. Chem. Soc. 122, 5114 (2000).

385 T. Ishikawa, Superbases for Organic Synthesis: Guanidines, Amidines, Phosphazenes and Related

Organocatalysts, Wiley (2009).

386 J.-M. Lehn, Science, 260, 1762 (1993).

387 C. G. Swain, E. C. Lupton, J. Am. Chem. Soc. 90, 4328 (1968).

388 W. F. Reynolds, R. D. Topsom, J. Org. Chem, 49, 1989 (1984).

389 A. J. Hoefnagel, W. Osterbeek, B. M. Wepster, J. Org. Chem, 49, 1993 (1984).

390 M. Charton, J. Org. Chem. 49, 1997 (1984).

391 C. G. Swain, J. Org. Chem, 49, 2005 (1984).

392 C. G. Swain, S. H. Unger, N. R. Rosenquist, M. S. Swain, J. Am. Chem. Soc. 105, 492 (1983).

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393 M. J. S. Dewar, J. Am. Chem. Soc. 106, 209 (1984).

394 W. T. Borden, R. J. Loncharich, K. N. Houk, Ann. Rev. Phys. Chem. 39, 213 (1988).

395 P. Merino, M. A. Chiacchio, L. Legnani, I. Delso, T. Tejero, Org. Chem. Frontiers, 4, 1541 (2017).

396 A. Moyano, J. Org. Chem. 54, 573 (1989).

397 W. L. Jorgensen, L .Salem, The Organic Chemist’s Book of Orbitals, Academic Press, New York (1973).

398 J. Shorter, Correlation Analysis in Organic Chemistry: An Introduction to Linear Free Energy Relationships.

Oxford University Press (1973).

399 R. W. Taft, J. Am. Chem. Soc. 74, 3120 (1952).


401 M. Tisler, Synthesis-Stuttgart 3, 123 (1973).

402 J. E. Torr, J. M. Large, P. N. Horton, M. B. Hursthouse, E. McDonald, Tetrahedron. Lett. 47, 31 (2006).

403 J. Suwiński, K. Świerczek, Tetrahedron 57, 1639 (2001).

404 C. W. Jefford, G. Bernardinelli, Y. Wang, D. C. Spellmeyer, A. Buda, K. N. Houk, J. Am. Chem. Soc. 114,

1157 (1992).

405 W. R. Dolbier, H. Koroniak, K. N. Houk, C. Sheu, Acc. Chem. Res. 29, 471 (1996).

406 R. A. Y. Jones, J. F. Bunnett. Nomenclature for Organic Chemical Transformations (Recommendations

1988), Pure Appl. Chem. 61, 725 (1989). http://publications.iupac.org/pac/pdf/1989/pdf/6104x0725.pdf

407 J. C. Polanyi, A. H. Zewail, Acc. Chem. Res. 28, 119 (1995).

408 H. B. Schlegel, J. Comput Chem. 3, 214 (1982).

409 R. E. Stanton, J. W. McIver, J. Am. Chem. Soc. 97, 3632 (1975).

410 R. P. Bell, The Tunnel Effect in Chemistry, Chapman and Hall, London (1980).

411 D. Ley, D. Gerbig, P. R. Schreiner, Org. Biomol. Chem. 10, 3781 (2012).

412 Z. R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 103, 3899 (2003).

413 D. Seebach, Angew. Chem. Int. Ed. 18, 239 (1979).

414 J. N. Israelachvili, Intermolecular and Surface Forces. 3rd ed., Academic Press, London (2011).

415 Y. Yukawa, Y. Tsuno, Bull. Chem. Soc. Japan, 32 , 971 (1959).

416 Y. Tsuno, M. Fujio, Adv. Phys. Org. Chem. 32, 267 (1999).

417 M. A. Paul, F. A. Long, Chem. Rev. 57, 1 (1957).

418 A. Schmidt, Adv. Heterocycl. Chem. 85, 67 (2003).

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For Peer Review Only1 GLOSSARY OF TERMS USED IN2 PHYSICAL ORGANIC CHEMISTRY345 Abstract. This Glossary contains definitions, explanatory notes, and sources for terms 6 used in physical organic chemistry. Its aim is to provide guidance on the terminology of 7 physical organic chemistry, with a view to achieving a consensus on the meaning and 8 applicability of useful terms and the abandonment of unsatisfactory ones. Owing to the 9 substantial progress in the field, this 2021 revision of the Glossary is much expanded

10 relative to the previous edition, and it includes terms from cognate fields. 111213 INTRODUCTION TO THE 2021 REVISION14 General Remarks1516 The first Glossary of Terms Used In Physical Organic Chemistry was published 17 in provisional form in 1979 [1] and in revised form in 1983, incorporating modifications 18 agreed to by IUPAC Commission III.2 (Physical Organic Chemistry) [2].19 A further revision was undertaken under the chairmanship of Paul Müller, which 20 was published in 1994 [3]. The work was coordinated with that of other Commissions 21 within the Division of Organic Chemistry. In 1999 Gerard P. Moss, with the assistance 22 of Charles L. Perrin, converted this glossary to a World Wide Web version [4]. The 23 Compendium of Chemical Terminology [5] (Gold Book) incorporated many of the terms 24 in the later version.

25 This Glossary has now been thoroughly revised and updated, to be made 26 available as a Web document. The general criterion adopted for the inclusion of a term 27 in this Glossary has been its wide use in the present or past literature of physical-28 organic chemistry and related fields, with particular attention to those terms that have 29 been ambiguous. It is expected that the terms in this Glossary will be incorporated within 30 the on-line version of the IUPAC Gold Book, which is the merged compendium of all 31 glossaries [5].

32 The aim of this Glossary is to provide guidance on the terminology of physical-33 organic chemistry, with a view to achieving a far-reaching consensus on the definitions 34 of useful terms and the abandonment of unsatisfactory ones. According to Antoine 35 Lavoisier "Comme ce sont les mots qui conservent les idées et qui les transmettent, il 36 en résulte qu'on ne peut perfectionner le langage sans perfectionner la science, ni la 37 science sans le langage,“ (As it is the words that preserve the ideas and convey them, 38 it follows that one cannot improve the language without improving science, nor improve

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1 science without improving the language.”) [6]. Our approach has been to take or update 2 entries from the previous glossary, whereas new terms were added by virtue of their 3 usage in the current literature and the diverse knowledge of the members of the Task 4 Force.

5 The Task Force is pleased to acknowledge the generous contributions of many 6 scientists who helped by proposing or defining new terms or by criticizing or modifying 7 existing ones. The following members of the Task Force have contributed to this 8 revision:9

10 Israel Agranat Charles L. Perrin, Chair11 Alessandro Bagno† Leo Radom12 Silvia E. Braslavsky Zvi Rappoport13 Pedro A. Fernandes Marie-Françoise Ruasse†

14 Jean-François Gal Hans-Ullrich Siehl15 Guy C. Lloyd-Jones Yoshito Takeuchi16 Herbert Mayr Thomas T. Tidwell17 Joseph R. Murdoch Einar Uggerud18 Norma Sbarbati Nudelman Ian H. Williams1920 († = deceased) 21 This version was prepared for publication by Charles L. Perrin, Silvia E. Braslavsky, 22 and Ian Williams. We especially note the able technical assistance of Gerard P. Moss 23 (Queen Mary University of London) and the advice of Christian Reichardt (Marburg). 24 We also thank the several reviewers whose comments improved the presentation, 25 especially Jan Kaiser (University of East Anglia).262728 Arrangement, Abbreviations, and Symbols29 The arrangement is alphabetical, terms with Greek letters following those in the 30 corresponding Latin ones. Italicized words in the body of a definition, as well as those 31 cited at the end, point to relevant cross-references. Literature references direct the 32 reader either to the original literature where the term was originally defined or to 33 pertinent references where it is used, including other IUPAC glossaries, where 34 definitions may differ from those here.35 Definitions of techniques not directly used for measurements in physical-organic 36 chemistry are not included here but may be consulted in specialized IUPAC texts, 37 including the IUPAC Recommendations for Mass Spectrometry [7], the Glossary of 38 Terms Used In Theoretical Organic Chemistry [8], the Glossary of Terms Used in 39 Photochemistry 3rd edition [9], the Glossary of Terms Used in Photocatalysis and 40 Radiation Catalysis [10] and the Basic Terminology of Stereochemistry [11].

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1 In accordance with IUPAC recommendations [12] the symbol ‡ to indicate 2 transition state ("double dagger") is used as a prefix to the appropriate quantities, e.g., 3 ∆‡G rather than ∆G‡. In equations including a logarithmic function the procedure 4 recommended by IUPAC was adopted, i.e., to divide each dimensioned quantity by its 5 units. Since this procedure often introduces a cluttering of the equations, we have in 6 some cases chosen a short-hand notation, such as ln {k(T)}, where the curly brackets 7 indicate an argument of dimension one, corresponding to k(T)/[k(T)], where the square 8 brackets indicate that the quantity is divided by its units, as recommended in [12].9

10 Note on the identification of new and/or revised terms.11 Terms that are found in the previous version of this Glossary [3,4] and currently 12 incorporated in the IUPAC “Gold Book” [5] are identified with a reference to the 13 Glossary of Terms used in Physical Organic Chemistry (1994), i.e., [3]are marked as 14 GB, whereas revised terms are designated as revGBrev[3]. . Revised terms from the 15 previous Glossary of Terms in Physical Organic Chemistry are labeled revPOC. Some 16 of them were not incorporated in the Gold Book. Minor changes such as better wording, 17 additional cross-referencing, or reorganization of the text without changing the concept 18 are, in general, not considered revisions. However, the improved version should replace 19 the older one in the “Gold Book”. New terms and terms from other IUPAC documents 20 are not identified as such.. In many cases new references have been added in the 21 definitions.222324

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1 A factor2 Arrhenius factor3 (SI unit same as rate constant: s–1 for first-order reaction).4 Pre-exponential factor in the Arrhenius equation for the temperature dependence of a 5 reaction rate constant.6 Note 1: According to collision theory, A is the frequency of collisions with the correct 7 orientation for reaction.8 Note 2: The common unit of A for second-order reactions is dm3 mol-1 s-1.9 See also A value, energy of activation, entropy of activation.

10 See [12,13].11 revGB-revPOC[3]1213 A value14 Steric substituent parameter expressing the conformational preference of an equatorial 15 substituent relative to an axial one in a monosubstituted cyclohexane.16 Note 1: This parameter equals rGº for the equatorial to axial equilibration, 17 in kJ mol–1. For example, ACH3 is 7.28 kJ mol–1, a positive value because an axial 18 methyl group is destabilized by a steric effect.19 Note 2: The values are also known as Winstein-Holness A values.20 See [3,14,15]. 21 GB2223 abstraction24 Chemical reaction or transformation, the main feature of which is the bimolecular 25 removal of an atom (neutral or charged) from a molecular entity.26 Examples:27 CH3COCH3 + (i-C3H7)2N– → [CH3COCH2]– + (i-C3H7)2NH28 (hydron abstraction from acetone)29 CH4 + Cl· → CH3· + HCl 30 (hydrogen atom abstraction from methane)31 See detachment.32 GB[3]3334 acceptor parameter (A, dimensionless)35 acceptor number (deprecated). Unit and dimension 1.36 Quantitative measure, devised by Gutmann [16] of the Lewis acidity of a solvent, based 37 on the 31P chemical shift of dissolved triethylphosphine oxide (triethylphosphane oxide).38 Note: The term acceptor number, designated by AN, is a misnomer and ought to be 39 called acceptor parameter, A, because it is an experimental value [17].40 rev[3]RevGB-revPOC

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12 acid3 Molecular entity or chemical species capable of donating a hydron (proton) or capable 4 of forming a bond with the electron pair of a Lewis base.5 See Brønsted acid, Lewis acid, Lewis base. 6 See also hard acid.7 GB[3]89 acidity

10 (1) Of a compound:11 Tendency of a Brønsted acid to act as a hydron (proton) donor, or tendency of a Lewis 12 acid to form Lewis adducts and -adducts.13 Note: Acidity can be quantitatively expressed by the acid dissociation constant of the 14 compound in water or in some other specified medium, by the association constants for 15 formation of Lewis adducts and -adducts, or by the enthalpy or Gibbs energy of 16 deprotonation in the gas phase.17 (2) Of a medium, usually one containing Brønsted acids:18 Tendency of the medium to hydronate a specific reference base.19 Note 1: The acidity of a medium is quantitatively expressed by the appropriate acidity 20 function.21 Note 2: Media having an acidity greater than that of 100 % H2SO4 are often called 22 superacids.23 See [18].24 rRev[3]GB-revPOC2526 acidity function27 Measure of the thermodynamic hydron-donating or -accepting ability of a solvent 28 system, or a closely related thermodynamic property, such as the tendency of the lyate 29 ion of the solvent system to form Lewis adducts.30 Note 1: Acidity functions are not unique properties of the solvent system alone but 31 depend on the solute (or family of closely related solutes) with respect to which the 32 thermodynamic tendency is measured.33 Note 2: Commonly used acidity functions are extensions of pH to concentrated acidic 34 or basic solutions. Acidity functions are usually established over a range of 35 compositions of such a system by UV/Vis spectrophotometric or NMR measurements 36 of the degree of hydronation (or Lewis adduct formation) for the members of a series of 37 structurally similar indicator bases (or acids) of different strength: the best known of 38 these is the Hammett acidity function Ho (for primary aromatic amines as indicator 39 bases).

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1 For detailed information on other acidity functions, on excess acidity, on the 2 evaluation of acidity functions, and on the limitations of the concept, see [19,20,21,22]. 3 [3]GB

456 activated complex7 See activated state.8 revGB-revPOCrev[3]9

10 activated state 11 In theories of unimolecular reactions an energized chemical species, often 12 characterized by the superscript ‡, where the excitation is specific and the molecule is 13 poised for reaction.14 Note 1: Often used as a synonym for activated complex or transition state, but not 15 restricted to transition-state theory.16 Note 2: This is distinct from an energized molecule, often characterized by the 17 superscript *, in which excitation energy is dispersed among internal degrees of 18 freedom.19 Note 3: This is not a complex according to the definition in this Glossary.20 See also transition state, transition structure.2122 activation energy 23 See energy of activation.24 See [12], section 2.12.25 [3]GB

26

27

28 activation strain model29 See distortion interaction model.3031 addition reaction32 Chemical reaction of two or more reacting molecular entities, resulting in a product 33 containing all atoms of all components, with formation of two chemical bonds and a net 34 reduction in bond multiplicity in at least one of the reactants.35 Note 1: The addition to an entity may occur at only one site (1,1-addition, insertion), 36 at two adjacent sites (1,2-addition) or at two non-adjacent sites (1,3- or 1,4-addition, 37 etc.). 38 Examples39

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1 R3C–H + H2C: → R3C–CH3 (1,1-addition of a carbene)2 Br2 + CH2=CH–CH=CH2 → BrCH2–CH(Br)–CH=CH2 (1,2-addition) + 3 BrCH2–CH=CH–CH2Br (1,4-addition)45 Note 2: This is distinguished from adduct formation, which is less specific about 6 bonding changes.7 Note 3: The reverse process is called an elimination reaction.8 See also cheletropic reaction, cycloaddition, insertion.9 revGB-revPOC

10 rev[3]1112 additivity principle13 Hypothesis that each of several structural features of a molecular entity makes an 14 independent, transferable, and additive contribution to a property of the substance 15 concerned.16 Note 1: More specifically, it is the hypothesis that each of the several substituent 17 groups in a parent molecule makes a separate and additive contribution to the standard 18 Gibbs energy change or Gibbs energy of activation corresponding to a particular 19 chemical reaction.20 Note 2: The enthalpies of formation of series of compounds can be described by 21 additivity schemes [23].22 Note 3: Deviations from additivity may be remedied by including terms describing 23 interactions between atoms or groups.24 See transferability [24,25].25 GB26 rev[3]2728 adduct29 New chemical species AB, each molecular entity of which is formed by direct 30 combination of two (or more) separate molecular entities A and B in such a way that 31 there is no loss of atoms from A or B.32 Example: adduct formed by interaction of a Lewis acid with a Lewis base:33 (CH3)3N+––BF3

34 Note 1: Stoichiometries other than 1:1 are also possible, e.g., a bis-adduct (2:1). 35 Note 2: An intramolecular adduct can be formed when A and B are groups contained 36 within the same molecular entity.37 Note 3: If adduct formation is prevented by steric hindrance, frustrated Lewis pairs 38 may result. 39 See also addition reaction, frustrated Lewis pair, Lewis adduct, Meisenheimer 40 adduct, -adduct.

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1 rRev[3]GB-revPOC23 agostic4 Feature of a structure in which a hydrogen atom is bonded to both a main-group atom 5 and a metal atom.6 Example, [(1–3-)-but-2-en-1-yl-2-C4,H4]( 5-cyclopentadienyl)cobalt(+1).789

101112 Note: The expression -hydrido-bridged is also used to describe the bonding 13 arrangement with a bridging hydrogen, but this usage is deprecated.14 See [26,27,28,29].15 revGB-revPOC16 rev[3]1718 alcoholysis19 Reaction with an alcohol solvent.20 Examples:2122 C3H5(OCOR)3 (triglyceride) + 3 CH3OH → C3H5(OH)3 (glycerol) + 3 RCOOH23 (CH3)3CCl + CH3OH → (CH3)3COCH3 + H+ + Cl–2425 See solvolysis.26 rev[3]27 revGB-revPOC2829 allotropes30 Different structural modifications of an element, with different bonding arrangements of 31 the atoms.32 Examples for carbon include diamond, fullerenes, graphite, and graphene.33 See [29].3435 allylic substitution reaction36 Substitution reaction on an allylic system with leaving group (nucleofuge) at position 1 37 and double bond between positions 2 and 3. The incoming group may become attached 38 to atom 1, or else the incoming group may become attached at position 3, with 39 movement of the double bond from 2,3 to 1,2.40 Example:

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1 CH3CH=CHCH2Br + HOAc → CH3CH=CHCH2OAc or CH3CH(OAc)CH=CH2

23 Note: This term can be extended to systems such as: 45 CH3CH=CH–CH=CH–CH2Br → CH3CH(OAc)–CH=CH–CH=CH2 + 6 CH3CH=CH–CH(OAc)–CH=CH2 + CH3CH=CH–CH=CH–CH2OAc78 and also to propargylic substitution, with a triple bond between positions 2 and 3 and 9 possible rearrangement to an allenic product.

10 rev[3]11 revGB-revPOC1213 alternant 14 Property of a conjugated system of electrons whose atoms can be divided into two 15 sets (marked as "starred" and "unstarred") so that no atom of either set is directly linked 16 to any other atom of the same set.17 Examples1819202122232425 Note 1: According to several approximate theories (including HMO theory), the 26 MOs for an alternant hydrocarbon are paired, such that for an orbital of energy + x 27 there is another of energy x. The coefficients of paired molecular orbitals at each 28 atom are the same, but with opposite sign for the unstarred atoms, and the electron 29 density at each atom in a neutral alternant hydrocarbon is unity.30 See [30,31,32].31 rev[3]32 revGB-revPOC 3334 ambident35 ambidentate, bidentate36 Characteristic of a chemical species whose molecular entities each possess two 37 alternative, distinguishable, and strongly interacting reactive centres, to either of which 38 a bond may be made.39 Note 1: Term most commonly applied to conjugated nucleophiles, for example an 40 enolate ion (which may react with electrophiles at either the -carbon atom or the

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1 oxygen) or a 4-pyridone, and also to the vicinally ambident cyanide ion and to cyanate 2 ion, thiocyanate ion, sulfinate ions, nitrite ion, and unsymmetrical hydrazines.3

4 56 Note 2: Ambident electrophiles are exemplified by carboxylic esters RC(O)OR', 7 which react with nucleophiles at either the carbonyl carbon or the alkoxy carbon, and 8 by Michael acceptors, such as enones, that can react at either the carbonyl or the -9 carbon.

10 Note 3: Molecular entities containing two non-interacting (or feebly interacting) 11 reactive centres, such as dianions of dicarboxylic acids, are not generally considered 12 to be ambident or bidentate and are better described as bifunctional.13 Note 4: The Latin root of the word implies two reactive centres, but the term has also 14 been applied to chemical species with more than two reactive centres, such as an acyl 15 thiourea, RCONHCSNHR', with nucleophilic O, S, and N. For such species the term 16 polydentate (or multidentate) is more appropriate.17 See [33,34,35,36]. 18 GB[3]1920 ambiphilic21 Both nucleophilic and electrophilic.22 Example:23 CH2=O24 See also amphiphilic, but the distinction between ambi (Latin: both) and amphi 25 (Greek: both) and the application to hydrophilic and lipophilic or to nucleophilic and 26 electrophilic is arbitrary.2728 aminoxyl29 Compound having the structure

30 R2N–O• R2N•+–O–

31 Note: The synonymous term 'nitroxyl radical' erroneously suggests the presence of 32 a nitro group; its use is deprecated.3334 amphiphilic35 Both hydrophilic and lipophilic, owing to the presence in the molecule of a large organic 36 cation or anion and also a long hydrocarbon chain (or other combination of polar and 37 nonpolar groups, as in nonionic surfactants)38 Examples:

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12 CH3(CH2)nCO2

–M+, CH3(CH2)nSO3–M+, CH3(CH2)nN(CH3)3

+X– (n > 7).34 Note: The presence of distinct polar (hydrophilic) and nonpolar (hydrophobic) regions 5 promotes the formation of micelles in dilute aqueous solution.6 See also ambiphilic.7 GB[3]89 amphiprotic (solvent)

10 Feature of a self-ionizing solvent possessing characteristics of both Brønsted acid and 11 base.12 Examples: H2O, CH3OH.13 GB[3]1415 amphoteric16 Property of a chemical species that can behave as either an acid or as a base.17 Examples: H2O, HCO3

– (hydrogen carbonate)18 Note: This property depends upon the medium in which the species is investigated. 19 For example HNO3 is an acid in water but becomes a base in H2SO4.20 rev[3]21 GB2223 anchimeric assistance24 neighbouring group participation25 GB[3]2627 anionotropy 28 Rearrangement or tautomerization in which the migrating group moves with its electron 29 pair.30 See [37].31 GB[3]3233 annelation34 Alternative, but less desirable term for annulation.35 Note: The term is widely used in German and French languages.36 GB[3]3738 annulation39 Transformation involving fusion of a new ring to a molecule via two new bonds.

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1 Note: Some authors use the term annelation for the fusion of an additional ring to an 2 already existing one, and annulation for the formation of a ring from an acyclic 3 precursor.4 See [38,39].5 See also cyclization.6 GB[3]78 annulene9 Conjugated monocyclic hydrocarbon of the general formula CnHn (n even) with the

10 maximum number of noncumulative double bonds and without side chains.11 Note: In systematic nomenclature an annulene may be named [n]annulene, where n 12 is the number of carbon atoms, e.g., [8]annulene for cycloocta-1,3,5,7-tetraene.13 See [40].14 See aromatic, Hückel (4n + 2) rule.15 GB [3] 1617 anomeric effect18 Tendency of an electronegative substituent alpha to a heteroatom in a six-membered 19 ring to prefer the axial position, as in the anomers of glucopyranose. 20 Note 1: The effect can be generalized to the conformational preference of an 21 electronegative substituent X to be antiperiplanar to a lone pair of atom Y in a system 22 R–Y–C–X with geminal substituents RY and X. 23 Note 2: The effect can be attributed, at least in part, to n-* delocalization of the lone 24 pair on Y into the C–X * orbital.25 See [11,41,42,43,44]. 26 GB2728 anomers29 The two stereoisomers (epimers) of a cyclic sugar or glycoside that differ only in the 30 configuration at C1 of aldoses or C2 of ketoses (the anomeric or acetal/ketal carbon).31 Example:3233343536 See [11].37 GB[3]3839 antarafacial, suprafacial

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1 Two spatially different ways whereby bonding changes can occur when a part of a 2 molecule undergoes two changes in bonding (bond-making or bond-breaking), either 3 to a common centre or to two related centres external to itself. These are designated 4 as antarafacial if opposite faces of the molecule are involved, and suprafacial if both 5 changes occur at the same face. The concept of face is clear from the diagrams in the 6 cases of planar (or approximately planar) frameworks with interacting orbitals.789

101112131415 For examples of the use of these terms see cycloaddition, sigmatropic rearrangement.16 See also anti, , .17 rev[3]18 revGB-revPOC1920 anti21 Stereochemical relationship of two substituents that are on opposite sides of a 22 reference plane, in contrast to syn, which means "on the same side".23 Note 1: Two substituents attached to atoms joined by a single bond are anti if the 24 torsion angle (dihedral angle) between the bonds to the substituents is greater than 90º, 25 in contrast to syn if it is less than 90º. 26 Note 2: A further distinction is made between antiperiplanar, synperiplanar, anticlinal 27 and synclinal.28 See [11,45,46,47].29 Note 3: When the terms are used in the context of chemical reactions or 30 transformations, they designate the relative orientation of substituents in the substrate 31 or product:32 (1) Addition to a carbon-carbon double bond:3334353637383940

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123 (2) Alkene-forming elimination: 4

567 (3) Aldol reaction (where syn and anti designate the relative orientation of CH3 8 and OH in the product)9

1011 Note 4: In examples (1) and (2) anti processes are always antarafacial, and syn 12 processes are suprafacial.13 Note 5: In the older literature the terms anti and syn were used to designate 14 stereoisomers of oximes and related compounds. That usage was superseded by the 15 terms trans and cis or E and Z.16 See [11].17 rev[3]18 GB-revPOC1920 anti-Hammond effect21 If a structure lying off the minimum-energy reaction path (MERP) is stabilized, the 22 position of the transition state moves toward that structure. 23 See Hammond Postulate, More O'Ferrall - Jencks diagram, perpendicular effect.24 rev[3]25 revGB-revPOC2627 aprotic (solvent)28 non-HBD solvent (non-Hydrogen-Bond Donating solvent)29 Solvent that is not capable of acting as a hydrogen-bond donor.

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1 Note: Although this definition applies to both polar and nonpolar solvents, the 2 distinction between HBD and non-HBD (or between protic and aprotic) is relevant only 3 for polar solvents.4 See HBD solvent.5 rev[3]6 revGB-revPOC789 aquation

10 Incorporation of one or more integral molecules of water into a species, with or without 11 displacement of one or more other atoms or groups.12 Example: The incorporation of water into the inner ligand sphere of an inorganic 13 complex.14 See [48].15 See also hydration.16 GB[3]1718 aromatic (adj.), aromaticity (n.)19 (1) Having a chemistry typified by benzene (traditionally).20 (2) Feature of a cyclically conjugated molecular entity whose electronic energy is 21 significantly lower or whose stability is significantly greater (owing to delocalization) 22 than that of a hypothetical localized structure (e.g., Kekulé structure). 23 Note 1: If the molecular entity is of higher energy or less stable than a hypothetical 24 localized structure, the entity is said to be antiaromatic.25 Note 2: A geometric parameter indicating bond-length equalization has been used 26 as a measure of aromatic character, as expressed in the harmonic oscillator model of 27 aromaticity [49]. 28 Note 3: The magnitude of the magnetically induced ring current, as observed 29 experimentally by NMR spectroscopy or by the calculated nucleus-independent 30 chemical shift (NICS) value, is another measure of aromaticity [50]. 31 Note 4: The terms aromatic and antiaromatic have been extended to describe the 32 stabilization or destabilization of transition states of pericyclic reactions. The 33 hypothetical reference structure is here less clearly defined, and use of the term is 34 based on application of the Hückel (4n + 2) rule and on consideration of the topology of 35 orbital overlap in the transition state, whereby a cycle with (4n+2) electrons and a 36 Möbius cycle with 4n electrons are aromatic. Reactions of molecules in the ground state 37 involving antiaromatic transition states proceed much less easily than those involving 38 aromatic transition states. 39 See [51,52,53]. See 19 articles in [54].40 See also Hückel (4n + 2) rule, Möbius aromaticity.

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1 [3]2 revGB-revPOC34 Arrhenius equation5 Empirical expression for the temperature dependence of a reaction rate constant k as6 k(T) = A exp(–EA/RT),7 with A the pre-exponential factor (Arrhenius A factor) and EA the Arrhenius energy of 8 activation, both considered to be temperature-independent.9 revGB[3]

1011 aryne12 Hydrocarbon derived from an arene by formal removal of two vicinal hydrogen atoms.13 Example: 1,2-didehydrobenzene (benzyne)14

151617 Note 1: 1,4-Didehydrobenzene ("p-benzyne diradical", structure above right). 18 Despite common usage this is not an aryne because there is no triple bond, and the 19 usage is deprecated.20 Note 2: Arynes are usually transient species.21 Note 3: The analogous heterocyclic compounds are called heteroarynes or 22 hetarynes.23 See [40,55]. 24 rev[3]25 revGB-revPOC2627 association28 Assembling of separate molecular entities into any aggregate, especially of oppositely 29 charged free ions into ion pairs or larger and not necessarily well-defined clusters of 30 ions held together by electrostatic attraction.31 Note: The term signifies the reverse of dissociation, but is not commonly used for the 32 formation of definite adducts by colligation or coordination.33 GB[3]3435 asymmetric induction

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1 Preferential formation in a chemical reaction of one enantiomer or diastereoisomer over 2 the other as a result of the influence of a chiral centre (stereogenic centre, chiral feature) 3 in the substrate, reagent, catalyst, or environment.4 Note: The term also refers to the formation of a new chiral centre or chiral feature 5 preferentially in one configuration under such influence.6 See [11].7 rev[3]8 revGB-revPOC9

10 atomic charge11 Net charge due to the nucleus and the average electronic distribution in a given region 12 of space. This region is considered to correspond to an atom in a molecular entity.13 Note 1: The boundary limits of an atom in a polyatomic molecular entity cannot be 14 defined, as they are not a quantum-mechanical observable. Therefore, different 15 conceptual schemes of dividing a molecule into individual atoms will result in different 16 atomic charges.17 Note 2: The atomic charge on an atom should not be confused with its formal charge. 18 For example, the N in NH4

+ is calculated to carry a net negative charge even though its 19 formal charge is +1, and each H is calculated to carry a net positive charge even though 20 its formal charge is 0.21 See [8].2223 atomic orbital24 Wavefunction that depends explicitly on the spatial coordinates of only one electron 25 around a single nucleus.26 See also molecular orbital.27 See [8].28 [3]2930 atropisomers31 Stereoisomers that are enantiomeric owing to hindered rotation about a single bond.32 Example (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl, BINAP):3334353637383940 See [11].

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12 attachment3 Transformation by which one molecular entity (the substrate) is converted into another 4 by the formation of one (and only one) two-centre bond between the substrate and 5 another molecular entity and which involves no other changes in connectivity in the 6 substrate.7 Example: formation of an acyl cation by attachment of carbon monoxide to a 8 carbenium ion (R+):9 R+ + CO → (RCO)+

10 See also colligation.11 revGB[3]1213 autocatalytic reaction14 Chemical reaction in which a product (or a reaction intermediate) also functions as 15 catalyst.16 Note: In such a reaction the observed rate of reaction is often found to increase with 17 time from its initial value.18 Example: acid-catalyzed bromination of acetophenone, PhCOCH3, because the 19 reaction generates HBr, which functions as a catalyst. 20 rev[3]21 revGB-revPOC2223 automerization24 degenerate rearrangement25 GB[3]2627 autoprotolysis28 Proton transfer reaction (hydron transfer reaction) between two identical amphoteric 29 molecules (usually of a solvent), one acting as a Brønsted acid and the other as a 30 Brønsted base.31 Example:32 2 H2O → H3O+ + HO–

33 [3]34 revGB-revPOC3536 autoprotolysis constant3738 Product of the activities of the species produced as the result of autoprotolysis.39 Example: The autoprotolysis constant for water, Kw (or KAP), is equal to the product 40 of the relative activities of the hydronium and hydroxide ions at equilibrium in pure water.

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12 Kw = a(H3O+)a(HO–) = 1.0 10–14 at 25 °C and 1 standard atmosphere.34 Note: Since the relative activity a(H2O) of water at equilibrium is imperceptibly 5 different from unity (with mole fraction as the activity scale and pure un-ionized water 6 as the standard state), the denominator in the expression for the thermodynamic 7 equilibrium constant Kw° for autoprotolysis has a value very close to 1.8

9 Kw° = a(H3O + )a(HO–)

a(H2O)2

1011 Furthermore, owing to the low equilibrium extent of dissociation, and if infinite dilution 12 is selected as the standard state, the activity coefficients of the hydronium and 13 hydroxide ions in pure water are very close to unity. This leads to the relative activities 14 of H3O+ and HO being virtually identical with the numerical values of their molar 15 concentrations, if the molar scale of activity is used and an activity of 1 mol dm3 is 16 chosen as the standard state. Thus 1718 Kw [H3O+][HO]1920 where [H3O+] and [HO] are the numerical values of the molar concentrations. The 21 autoprotolysis constant has the unit (and dimension) 1 because each relative activity 22 has dimension 1, being a quotient of an absolute activity (including units) divided by a 23 common unit standard state activity (including units).24 See [56,57].25 rev[3]26 revGB-revPOC2728 (alpha)29 (1) Designation applied to the carbon to which a functional group is attached.30 (2) In carbohydrate nomenclature a stereochemical designation of the configuration at 31 the anomeric carbon.32 (3) Parameter in a Brønsted relation expressing the sensitivity of the rate of protonation 33 to acidity. 34 (4) Parameter in Leffler's relation expressing the sensitivity of changes in Gibbs 35 activation energy to changes in overall Gibbs energy for an elementary reaction.36 See [11].37 rev[3]38 revGB39

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1 -effect2 Positive deviation of an nucleophile (one bearing an unshared pair of electrons on an 3 atom adjacent to the nucleophilic site) from a Brønsted-type plot of lg {knuc} vs. pKa. The 4 argument in the lg function should be of dimension 1. Thus, reduced rate coefficients 5 should be used. Here {knuc} = knuc/[knuc] is the reduced knuc.6 Note 1: More generally, it is the influence on the reactivity at the site adjacent to the 7 atom bearing a lone pair of electrons.8 Note 2: The term has been extended to include the effect of any substituent on an 9 adjacent reactive centre, e.g., the -silicon effect.

10 See [58,59,60]. 11 See also Brønsted relation.12 rev[3]13 revGB1415 -elimination16 1,1-elimination17 Transformation of the general type1819 RR'ZXY → RR'Z + XY (or X + Y, or X+ + Y–)2021 where the central atom Z is commonly carbon.22 See also elimination.23 revGB[3]2425 Baldwin's rules26 Set of empirical rules for closures of 3- to 7-membered rings.27 Note: The favoured pathways are those in which the length and nature of the linking 28 chain enable the terminal atoms to achieve the proper geometry and orbital overlaps 29 for reaction.30 Example: Hex-5-en-1-yl radical undergoes 5-exo-trig cyclization to cyclopentylmethyl 31 radical rather than 6-endo-trig cyclization to cyclohexyl radical.32

33 CH2

CH2 not

3435 See [61,62]. 36 rev[3]37 revGB-revPOC 38

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1 base2 Chemical species or molecular entity having an available pair of electrons capable of 3 forming a bond with a hydron (proton) (see Brønsted base) or with the vacant orbital of 4 some other species (see Lewis base).5 See also hard base, superbase.6 GB[3]78 basicity 9 (1) Tendency of a Brønsted base to act as hydron (proton) acceptor.

10 Note 1: The basicity of a chemical species is normally expressed by the acidity or 11 acid-dissociation constant of its conjugate acid (see conjugate acid-base pair). 12 Note 2: To avoid ambiguity, the term pKaH should be used when expressing basicity 13 by the acid-dissociation constant of its conjugate acid. Thus the pKaH of NH3 is 9.2, 14 while its pKa, expressing its acidity, is 38. 15 (2) Tendency of a Lewis base to act as a Lewis acid acceptor.16 Note 3: For Lewis bases, basicity is expressed by the association constants of Lewis 17 adducts and -adducts, or by the enthalpy of an acid/base reaction.18 Note 4: Spectroscopic shifts induced by acid/base adduct formation can also be used 19 as a measure of the strength of interaction.20 See [63].21 rev[3]22 revGB-revPOC2324 bathochromic shift (effect)25 Shift of a spectral band to lower frequencies (longer wavelengths).26 Note: This is informally referred to as a red shift and is opposite to a hypsochromic 27 shift ("blue shift"), but these historical terms are discouraged because they apply only 28 to visible transitions.29 See [9].30 GB[3]3132 Bell-Evans-Polanyi principle33 Linear relation between energy of activation (EA) and enthalpy of reaction (rH), 34 sometimes observed within a series of closely related reactions.3536 EA = a + b rH 3738 See [64,65,66,67].39 rev[3]40 revGB-revPOC

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12 benzyne3 1,2-Didehydrobenzene (a C6H4 aryne derived from benzene) and its derivatives formed 4 by substitution.5 Note: The terms m- and p-benzyne are occasionally used for 1,3- and 1,4-6 didehydrobenzene, respectively, but these are incorrect because there is no triple bond.7 See [40,55].8 GB[3]9

10 bidentate11 Feature of a ligand with two potential binding sites.1213 bifunctional catalysis14 Catalysis (usually of hydron transfer) by a chemical species involving a mechanism in 15 which two functional groups are implicated in the rate-limiting step, so that the 16 corresponding catalytic coefficient is larger than that expected for catalysis by a 17 chemical species containing only one of these functional groups.18 Example: Hydrogen carbonate is a particularly effective catalyst for the hydrolysis of 19 4-hydroxybutyranilide (N-phenyl-4-hydroxybutanamide), because it catalyzes the 20 breakdown of the tetrahedral intermediate to expel aniline: 21

222324 Note: The term should not be used to describe a concerted process involving the 25 action of two different catalysts.26 See [68,69,70,71,72].27 rev[3]28 revGB-revPOC2930 bifurcation31 Feature on a potential-energy surface whereby a minimum-energy reaction path 32 (MERP) emanating from a saddle point (corresponding to a transition structure) splits

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1 in two and leads to alternative products without intervening minima or secondary 2 barriers to overcome. A bifurcation arises when the curvature of the surface in a 3 direction perpendicular to the MERP becomes zero and then negative; it implies the 4 existence of a lower-energy transition structure with a transition vector orthogonal to 5 the original MERP.6 See [73,74].78 bimolecular9 See molecularity.

10 GB[3]1112 binding site13 Specific region (or atom) in a molecular entity that is capable of entering into a 14 stabilizing interaction with another atomic or molecular entity.15 Example: an active site in an enzyme that interacts with its substrate.16 Note 1: Typical modes of interaction are by covalent bonding, hydrogen bonding, 17 coordination, and ion-pair formation, as well as by dipole-dipole interactions, dispersion 18 forces, hydrophobic interactions, and desolvation.19 Note 2: Two binding sites in different molecular entities are said to be complementary 20 if their interaction is stabilizing.21 GB[3]2223 biradical24 diradical25 See [9].26 rev[3]27 GB-revPOC2829 blue shift30 Informal expression for hypsochromic shift, but this historical term is discouraged 31 because it applies only to visible transitions.32 rev[3]33 revGB3435 Bodenstein approximation36 See steady state.37 GB[3]3839 bond

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1 Balance of attractive and repulsive forces between two atoms or groups of atoms, 2 resulting in sufficient net stabilization to lead to the formation of an aggregate 3 conveniently considered as an independent molecular entity.4 Note: The term usually refers to the covalent bond.5 See [75]. 6 See also agostic, coordination, hydrogen bond, multi-centre bond.7 rev[3]8 revGB-revPOC9

10 bond dissociation11 See heterolysis, homolysis.12 Note: In ordinary usage the term refers to homolysis. If not, it should be specified as 13 heterolytic.14 GB[3]1516 bond-dissociation energy17 De

18 (SI unit: kJ mol–1)19 Energy required to break a given bond of some specific molecular entity by homolysis 20 from its potential-energy minimum.21 Note: This is the quantity that appears in the Morse potential.22 See also bond-dissociation enthalpy.23 rev[3]24 revGB-revPOC2526 bond-dissociation enthalpy27 DH0, dissH0

28 (SI unit: kJ mol–1)29 Standard molar enthalpy required to break a given bond of some specific molecular 30 entity by homolysis.31 Example: For CH4 → CH3· + H· the bond-dissociation enthalpy is symbolized as 32 DH0(CH3–H).33 Note: Although DH0 is commonly used, dissH0 is more consistent with the notation 34 for other thermodynamic quantities.35 See also bond-dissociation energy, bond energy, heterolytic bond-dissociation 36 enthalpy.3738 bond energy39 D0

40 (SI unit: kJ mol–1)

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1 Enthalpy of bond dissociation at 0 K.2 See bond-dissociation enthalpy.3 rev[3]4 revGB-revPOC56 bond enthalpy (mean bond enthalpy, mean bond energy)7 Average value of the gas-phase bond-dissociation enthalpies (usually at a temperature 8 of 298 K) for all bonds of the same type within the same chemical species.9 Example: For methane, the mean bond enthalpy is 415.9 kJ mol–1, one-fourth the

10 enthalpy of reaction for11 CH4(g) → C(g) + 4 H(g)12 Note: More commonly, tabulated mean bond energies (which are really enthalpies) 13 are values of bond enthalpies averaged over a number of selected chemical species 14 containing that type of bond, such as 414 kJ mol–1 for C–H bonds in a group of R3CH 15 (R = H, alkyl).16 See [76]. 1718 bond order19 Theoretical index of the degree of bonding between two atoms, relative to that of a 20 normal single bond, i.e., the bond provided by one localized electron pair.21 Example: In ethene the C–C bond order is 2, and the C–H bond order is 1.22 Note 1: In valence-bond theory it is a weighted average of the bond orders between 23 the respective atoms in the various resonance forms. In molecular-orbital theory it is 24 calculated from the weights of the atomic orbitals in each of the occupied molecular 25 orbitals. For example, in valence-bond theory the bond order between adjacent carbon 26 atoms in benzene is 1.5; in Hückel molecular orbital theory it is 1.67.27 Note 2: Bond order is often derived from the electron distribution.28 Note 3: The Pauling bond order n (as often used in the bond-energy-bond-order 29 model) is a simple function of change in bond length d, where the value of the coefficient 30 c is often 0.3 Å (for n > 1) or 0.6 Å (for n < 1).3132 n = exp[(d1 – dn)/c]33 rev[3]34 revGB-revPOC3536 bond-energy-bond-order model (BEBO):37 Empirical procedure for estimating activation energy, involving relationships among 38 bond length, bond-dissociation energy, and bond order.39 See [13,77]. 40 GB

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12 bond-stretch isomers3 Two (or more) molecules with the same spin multiplicity but with different lengths for 4 one or more bonds.5 Note: This feature arises because the potential-energy surface, which describes how 6 the energy of the molecule depends on geometry, shows two (or more) minima that are 7 not merely symmetry-related. 8 See [78,79,80,81].9

10 borderline mechanism11 Mechanism intermediate between two extremes, for example a nucleophilic substitution 12 intermediate between SN1 and SN2, or intermediate between electron transfer and SN2.13 GB[3]1415 Born-Oppenheimer approximation16 Representation of the complete wavefunction as a product of electronic and nuclear 17 parts, (r,R) = el(r,R) nuc(R), so that the two wavefunctions can be determined 18 separately by solving two different Schrödinger equations.19 See [8].20 GB2122 Bredt's rule23 Prohibition of placing a double bond with one terminus at the bridgehead atom of a 24 polycyclic system unless the rings are large enough to accommodate the double bond 25 without excessive strain.26 Example: Bicyclo[2.2.1]hept-1-ene (A), which is capable of existence only as a 27 transient species, although its higher homologues, bicyclo[3.3.1]non-1-ene (B) and 28 bicyclo[4.2.1]non-1(8)-ene (C), with double bond at the bridgeheads, have been 29 isolated.3031323334 A B C 3536 See [82,83,84,85].37 For limitations see [86].38 Note: For an alternative formulation, based on the instability of a trans double bond 39 in a small ring (fewer than 8 atoms), see [87].40 rev[3]

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1 revGB-revPOC23 bridged carbocation4 Carbocation (real or hypothetical) in which there are two (or more) carbon atoms that 5 could in alternative Lewis formulas be designated as carbenium centres but which is 6 instead represented by a structure in which a group (a hydrogen atom or a hydrocarbon 7 residue, possibly with substituents in non-involved positions) bridges these potential 8 carbenium centres.9 Note: Electron-sufficient bridged carbocations are distinguished from electron-

10 deficient bridged carbocations. Examples of the former, where the bridging uses 11 electrons, are phenyl-bridged ions (for which the trivial name phenonium ion has been 12 used), such as A. These ions are straightforwardly classified as carbenium ions. The 13 latter type of ion necessarily involves three-centre bonding because the bridging uses 14 σ electrons. The hydrogen-bridged carbocation B contains a two-coordinate hydrogen 15 atom, whereas structures C, D, and E (the 2-norbornyl cation) contain five-coordinate 16 carbon atoms. 17

181920 See [88].21 See also carbonium ion, multi-centre bond, neighbouring group participation.22 For the definitive X-ray structure of the norbornyl cation see [89].23 rev[3]24 revGB-revPOC25262728 bridgehead (atom)29 Atom that is part of two or more rings in a polycyclic molecule and that is separated 30 from another bridgehead atom by bridges all of which contain at least one other atom.31 Example: C1 and C4 in bicyclo[2.2.1]heptane, but not C4a and C8a in 32 decahydronaphthalene.3334 bridging ligand35 Ligand attached to two or more, usually metallic, central atoms.36 See [29].

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1 GB[3]23 Brønsted acid (Brönsted acid)4 Molecular entity capable of donating a hydron (proton) to a base (i.e., a hydron donor), 5 or the corresponding chemical species.6 Examples: H2O, H3O+, CH3COOH, H2SO4, HSO4

–, HCl, CH3OH, NH3.7 See also conjugate acid-base pair.8 GB[3]9

10 Brønsted base (Brönsted base)11 Molecular entity capable of accepting a hydron (proton) from a Brønsted acid (i.e., a 12 hydron acceptor), or the corresponding chemical species.13 Examples: HO–, H2O, CH3CO2

–, HSO4–, SO4

2–, Cl–, CH3O–, NH2–.

14 See also conjugate acid-base pair.15 GB[3]1617 Brønsted relation (Brönsted relation)18 Either of the equations1920 lg({kHA}/p) = C + lg(q{KHA}/p)2122 lg({kA}/q) = C – lg(q{KHA}/p)2324 where, , and C are constants for a given reaction series ( and are called Brønsted 25 exponents or Brønsted parameters). The arguments in the lg functions should be of 26 dimension 1. Thus, reduced rate coefficients should be used: {kA} = kA/[kA] and {kHA} = 27 kHA/[kHA] , which are the reduced catalytic coefficients of reactions whose rates depend 28 on the concentrations of acid HA or of its conjugate base A–, {KHA} = KHA/[KHA] is the 29 reduced acid dissociation constant of HA, p is the number of equivalent acidic protons 30 in HA, and q is the number of equivalent basic sites in A–. The chosen values of p and 31 q should always be specified. (The charge designations of HA and A– are only 32 illustrative.). 33 Note1: The equations are often written without reduced variables, whereupon the 34 slope or , obtained from a graph or least-squares analysis, is correct because it is 35 the dimensionless derivative of a logarithmic quantity, is of dimension 1.36 Note 2: The Brønsted relation is often termed the Brønsted catalysis law. Although 37 justifiable on historical grounds, use of this name is not recommended, since Brønsted 38 relations are known to apply to many uncatalysed and pseudo-catalysed reactions 39 (such as simple proton [hydron] transfer reactions).

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1 Note 3: The term pseudo-Brønsted relation is sometimes used for reactions that 2 involve nucleophilic catalysis instead of acid-base catalysis. Various types of Brønsted 3 parameters have been proposed such as nuc or lg for nucleophile or leaving group, 4 respectively. 5 See also linear free-energy relation (linear Gibbs energy relation).6 rev[3]7 revGB-revPOC89 Bunnett-Olsen equations

10 Relations between lg([SH+]/[S]) + Ho and Ho + lg{[H+]} for base S in aqueous acid 11 solutions, where Ho is Hammett's acidity function and Ho + lg{[H+]} represents the 12 activity function lg({S} {H+} / {SH+}) for the nitroaniline reference bases to build Ho. and 13 where is an empirical parameter that is determined by the slope of the linear 14 correlation of lg([SH+]/[S]) – lg{[H+]} vs. Ho + lg{[H+]}.1516 lg([SH+]/[S]) – lg{[H+]} = ( – 1)(Ho + lg{[H+]}) + pKSH+

1718 lg([SH+]/[S]) + Ho = (Ho + lg{[H+]}) + pKSH+

1920 Arguments in the lg functions should be of dimension 1. Thus, concentrations should 21 be divided by the respective unit (unless they are eliminated as in the ratio of two 22 concentrations), i.e., the reduced quantity should be used, indicated by curly brackets.23 Note 1: These equations avoid using (or defining) an acidity function for each family 24 of bases, including those for which such a definition is not possible. In many cases, 25 (or – 1) values for base families defining an acidity function are very similar. Broadly, 26 the value of is related to the degree of solvation of SH+.27 Note 2: These equations are obsolete, and the Cox-Yates equation, with the 28 equivalent parameter m* (= 1 – ), is now preferred.29 See [21,90]. 30 See also Cox-Yates equation.31 rev[3]32 revGB-revPOC3334 , nuc, lg

35 Parameter in a Brønsted relation expressing the sensitivity of the rate of deprotonation 36 to basicity.37 Note: nuc and lg are used to correlate nucleophilic reactivity and leaving-group 38 ability, respectively.3940 -elimination

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1 1,2-elimination2 Transformation of the general type34 XABY → A=B + XY (or X + Y, or X+ + Y–)56 where the central atoms A and B are commonly, but not necessarily, carbon.7 See also elimination.89 cage

10 Aggregate of molecules, generally solvent molecules in the condensed phase, that 11 surrounds fragments formed by thermal or photochemical dissociation.12 Note: Because the cage hinders the separation of the fragments by diffusion, they 13 may preferentially react with one another ("cage effect") although not necessarily to re-14 form the precursor species.15 Example:

1617 See also geminate recombination.18 GB[3]1920 cage compound21 Polycyclic compound capable of encapsulating another compound22 Example (adamantane, where the central cavity is large enough to encapsulate He, 23 Ne, or Na+):242526272829 Note: A compound whose cage is occupied is called an inclusion complex.30 See [91,92].31 rev[3]32 revGB-revPOC333435 canonical form36 resonance form37 rev[3]38 revGB-revPOC3940 captodative effect

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1 Combined action of an electron-withdrawing ("captor") substituent and an electron-2 releasing ("dative") substituent, both attached to a radical centre, on the stability of a 3 carbon-centred radical.4 Example:56789

10 See [93,94,95].11 rev[3]12 revGB-revPOC1314 carbanion15 Generic name for an anion containing an even number of electrons and having an 16 unshared pair of electrons on a carbon atom which satisfies the octet rule and bears a 17 negative formal charge in its Lewis structure or in at least one of its resonance forms.18 Examples:

19H C C CH3 CH2

O

CH3 CH2

O

2021 See also radical ion.22 See [40,96].23 rev[3]24 revGB-revPOC2526 carbene27 Generic name for the species H2C: ("methylidene") and substitution derivatives thereof, 28 containing an electrically neutral bivalent carbon atom with two nonbonding electrons.29 Note 1: The nonbonding electrons may have antiparallel spins (singlet state) or 30 parallel spins (triplet state).31 Note 2: Use of the alternative name "methylene" as a generic term is not 32 recommended.33 See [40].34 See also diradical.35 GB[3]3637 carbenium ion

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1 Generic name for a carbocation whose electronic structure can be adequately 2 described by two-electron-two-centre bonds.3 Note 1: The name implies a hydronated carbene or a substitution derivative thereof.4 Note 2: The term is a replacement for the previously used term carbonium ion, which 5 now specifies a carbocation with penta- or higher-coordinate carbons. The names 6 provide a useful distinction between tricoordinate and pentacoordinate carbons.7 Note 3: To avoid ambiguity, the name should be avoided as the root for the 8 nomenclature of carbocations. For example, the term "ethylcarbenium ion" might refer 9 to either CH3CH2

+ (ethyl cation, ethylium) or CH3CH2CH2+ (propyl cation, propylium).

10 See [40,97].11 rev[3]12 GB-revPOC1314 carbenoid15 Carbene-like chemical species but with properties and reactivity differing from the free 16 carbene, arising from additional substituents bonded to the carbene carbon.17 Example: R1R2C(Cl)M (M = metal)18 rev[3]19 revGB2021 carbocation22 Positive ion containing an even number of electrons and with a significant portion of the 23 excess positive charge located on one or more carbon atoms.24 Note 1: This is a general term embracing carbenium ions, all types of carbonium 25 ions, vinyl cations, etc.26 Note 2: Carbocations may be named by adding the word "cation" to the name of the 27 corresponding radical [97,98].28 Note 3: Such names do not imply structure (e.g., whether three-coordinated or five-29 coordinated carbon atoms are present) [98].30 See also bridged carbocation, radical ion.31 See [40].32 GB[3]3334 carbonium ion35 (1) Carbocation that contains at least one carbon atom with a coordination number of 36 five or greater.37 (2) Carbocation whose structure cannot adequately be described by only two-electron 38 two-centre bonds.39 Example: methanium (CH5

+).

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1 Note 1: In most of the earlier literature this term was used for all types of 2 carbocations, including those that are now defined as a (tricoordinate) carbenium ion.3 Note 2: To avoid ambiguity, the term should be avoided as the root for the 4 nomenclature of carbocations. For example, the name "ethylcarbonium ion" might refer 5 to either CH3CH2

+ (ethyl cation) or CH3CH2CH2+ (propyl cation).

6 See [98,99].7 rev[3]8 revGB-revPOC9

10 carbyne11 methylidyne 12 Generic name for the species HC·: and substitution derivatives thereof, such as 13 EtOCO–C·: (2-ethoxy-2-oxoethylidyne), containing an electrically neutral univalent 14 carbon atom with three non-bonding electrons.15 Note: Use of the alternative name "methylidyne" as a generic term is not 16 recommended.17 GB[3]1819 Catalán solvent parameters20 Quantitative measure of solvent polarity, based on the solvent’s hydrogen-bond-donor 21 ability, hydrogen-bond-acceptor ability, polarizability, and dipolarity.22 See [100,101].23 See also solvent parameter.2425 catalyst26 Substance that increases the rate of a chemical reaction (owing to a change of 27 mechanism to one having a lower Gibbs energy of activation) without changing the 28 overall standard Gibbs energy change (or position of equilibrium).29 Note 1: The catalyst is both a reactant and product of the reaction, so that there is 30 no net change in the amount of that substance.31 Note 2: At the molecular level, the catalyst is used and regenerated during each set 32 of microscopic chemical events leading from a molecular entity of reactant to a 33 molecular entity of product.34 Note 3: The requirement that there be no net change in the amount of catalyst is 35 sometimes relaxed, as in the base catalysis of the bromination of ketones, where base 36 is consumed, but this is properly called pseudo-catalysis.37 Note 4: Catalysis can be classified as homogeneous, in which only one phase is 38 involved, and heterogeneous, in which the reaction occurs at or near an interface 39 between phases.

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34

1 Note 5: Catalysis brought about by one of the products of a reaction is called 2 autocatalysis.3 Note 6: The terms catalyst and catalysis should not be used when the added 4 substance reduces the rate of reaction (see inhibition).5 Note 7: The above definition is adequate for isothermal-isobaric reactions, but under 6 other experimental conditions the state function that is lowered by the catalyst is not the 7 Gibbs activation energy but the quantity corresponding to those conditions (e.g., the 8 Helmholtz energy under isothermal-isochoric conditions).9 See [13].

10 See also autocatalytic reaction, bifunctional catalysis, catalytic coefficient, electron-11 transfer catalysis, general acid catalysis, general base catalysis, intramolecular 12 catalysis, micellar catalysis, Michaelis-Menten kinetics, phase-transfer catalysis, 13 pseudo-catalysis, rate of reaction, specific catalysis.14 rev[3]15 revGB-revPOC1617 catalytic antibody (abzyme)18 monoclonal antibody with enzymatic activity19 Note 1: A catalytic antibody acts by binding its antigen and catalyzing a chemical 20 reaction that converts the antigen into desired products. Despite the existence of natural 21 catalytic antibodies, most of them were specifically designed to catalyze desired 22 chemical reactions. 23 Note 2: Catalytic antibodies are produced through immunization against a transition-24 state analogue for the reaction of interest. The resulting antibodies bind strongly and 25 specifically the transition-state analogue, so that they become catalysts for the desired 26 reaction.27 Note 3: The concept of catalytic antibodies and the strategy for obtaining them were 28 advanced by W. P. Jencks [102]. The first catalytic antibodies were finally produced in 29 1986 [103,104].3031 catalytic coefficient32 If the rate of reaction v is expressible in the form3334 v = (k0 + ki[Ci]ni) [A]a [B]b...3536 where A, B, ... are reactants and Ci represents one of a set of catalysts, then the 37 proportionality factor ki is the catalytic coefficient of the particular catalyst Ci.38 Note: Normally the partial order of reaction (ni) with respect to a catalyst will be unity, 39 so that ki is an (a + b+...+1)th-order rate coefficient.40 GB[3]

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1234 catalytic cycle5 Sequence of reaction steps in the form of a loop. One step is binding of a reactant to 6 the active catalyst (sometimes formed from a precatalyst), and another step is the 7 release of product and regeneration of catalyst.

8 Example: A + B → C, catalyzed by X formed from precatalyst X'.

910 cation/ interaction 11 Noncovalent attractive force between a positive ion (metal cation, protonated Brønsted 12 base, etc.) and a electron system.13 See [105].1415 chain reaction16 Reaction in which one or more reactive reactive intermediates (frequently radicals) are 17 continuously regenerated through a repetitive cycle of elementary steps ("propagation 18 steps").19 Example: Chlorination of methane by a radical mechanism, where Cl· is continually 20 regenerated in the chain-propagation steps:21 Cl· + CH4 → HCl + H3C·22 H3C· + Cl2 → CH3Cl + Cl·23 Note: In chain polymerization reactions, reactive intermediates of the same types, 24 generated in successive steps or cycles of steps, differ in molecular mass, as in25 RCH2C·HPh + H2C=CHPh → RCH2CHPhCH2C·HPh26 See [106].27 See also chain transfer, initiation, termination.28 GB[3]2930 chain transfer

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1 Chemical reaction during a chain polymerization in which the active centre is transferred 2 from the growing macromolecule to another molecule or to another site on the same 3 molecule, often by abstraction of an atom by the radical end of the growing 4 macromolecule.5 Note 1: The growth of the polymer chain is thereby terminated but a new radical, 6 capable of chain propagation and polymerization, is simultaneously created. For the 7 example of alkene polymerization cited for a chain reaction, the reaction

8 RCH2C·HPh + CCl4 → RCH2CHClPh + Cl3C·9 represents a chain transfer, the radical Cl3C· inducing further polymerization:

10 Note 2: Chain transfer also occurs in other chain reactions, such as cationic or 11 anionic polymerization, in which case the abstraction is by the reactive cationic or 12 anionic end of the growing chain.13 See [106].14 See also telomerization.15 GB[3]1617 charge density18 See electron density.19 See [12].20 GB[3]2122 charge-transfer (CT) complex23 Ground-state adduct that exhibits an electronic absorption corresponding to light-24 induced transfer of electronic charge from one region of the adduct to another.25 See [9].26 rev[3]27 revPOC2829 chelation30 Formation or presence of bonds (or other attractive interactions) between a single 31 central atom (or ion) and two or more separate binding sites within the same ligand.32 Note 1: A molecular entity in which there is chelation (and the corresponding 33 chemical species) is called a chelate, while the species that binds to the central atom 34 is called a chelant.35 Note 2: The terms bidentate, tridentate, ... multidentate are used to indicate the 36 number of potential binding sites of the ligand, at least two of which must be used by 37 the ligand in forming a chelate. For example, the bidentate ethylenediamine forms a 38 chelate with Cu+2 in which both nitrogen atoms of ethylenediamine are bonded to 39 copper.40 Note 3: The use of the term is often restricted to metallic central atoms or ions.

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1 Note 4: The phrase "separate binding sites" is intended to exclude cases such as 2 [PtCl3(CH2=CH2)]–, ferrocene, and (benzene)tricarbonylchromium, in which ethene, the 3 cyclopentadienyl group, and benzene, respectively, are considered to present single 4 binding sites to the respective metal atom.5 See also ambident, cryptand.6 See also [29].7 GB[3]89 cheletropic reaction

10 Cycloaddition across the terminal atoms of a π system with formation of two new 11 bonds to a single atom of a monocentric reagent. There is formal loss of one bond in 12 the substrate and an increase in coordination number of the relevant atom of the 13 reagent.14 Example: addition of sulfur dioxide to butadiene:

1516 Note: The reverse of this type of reaction is designated "cheletropic elimination". 17 See [107].18 GB[3]1920 chelotropic reaction21 Alternative (and etymologically more correct) name for cheletropic reaction.22 See [51].23 GB[3]2425 chemical flux, 26 Unidirectional rate of reaction, applicable to the progress of component reaction steps 27 in a complex system or to the progress of reactions in a system at dynamic equilibrium 28 (in which there are no observable concentration changes with time), excluding the 29 reverse reaction and other reaction steps.30 Note 1: Chemical flux is a derivative with respect to time, and has the dimensions of 31 amount of substance per volume transformed per time.32 For example, for the mechanism33 A + B C⇄34 C + D → E35 1 is the chemical flux due to forward reaction step 1, or the rate of formation of C or 36 the rate of loss of A or B due to that step, and similarly for–1 and 2.

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1 Note 2: The sum of all the chemical fluxes leading to C is designated the "total 2 chemical flux into C" (symbol C), and the sum of all the chemical fluxes leading to 3 destruction of C is designated the "total chemical flux out of C" (symbol –C), and 4 similarly for A and B. It then follows for this example that C = 1 and –C =–1 + 2,5 Note 3: The net rate of appearance of C is then given by67 d[C]/dt = C – –C 89 Note 4: In this system 1 (or C) can be regarded as the hypothetical rate of

10 formation of C due to the single (unidirectional) reaction 1 proceeding in the assumed 11 absence of all other reactions, and –C can be regarded as the hypothetical rate of 12 destruction of C due to the two (unidirectional) reactions –1 and 2.13 Note 5: Even when there is no net reaction, chemical flux can often be measured by 14 NMR methods. 15 See [1]. 16 See also order of reaction, rate-limiting step, steady state.17 rev[3]18 revGB-revPOC1920 chemical reaction21 Process that results in the interconversion of chemical species.22 Note 1: This definition includes experimentally observable interconversions of 23 conformers and degenerate rearrangements.24 Note 2: Chemical reactions may be elementary reactions or stepwise reactions.25 Note 3: Detectable chemical reactions normally involve sets of molecular entities, as 26 indicated by this definition, but it is often conceptually convenient to use the term also 27 for changes involving single molecular entities (i.e., "microscopic chemical events"), 28 whose reactions can now be observed experimentally.29 See also identity reaction.30 rev[3]31 revGB-revPOC3233 chemical relaxation34 Passage of a perturbed system toward or into chemical equilibrium.35 Note 1: A chemical reaction at equilibrium can be disturbed from equilibrium by a 36 sudden change of some external parameter such as temperature, pressure, or electric-37 field strength.38 Note 2: In many cases, and in particular when the displacement from equilibrium is 39 slight, the progress of the system towards equilibrium can be expressed as a first-order 40 process

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12 c(t) – (ceq)2 = [(ceq)1 – (ceq)2] exp(–t/)34 where (ceq)1 and (ceq)2 are the equilibrium amount concentrations of one of the chemical 5 species before and after the change in the external parameter, and c(t) is its amount 6 concentration at time t. The time parameter, called relaxation time, is related to the 7 rate coefficients of the chemical reaction involved. Such measurements are commonly 8 used to follow the kinetics of very fast reactions.9 Note 3: Relaxation, or the passage toward equilibrium, is more general than chemical

10 relaxation, and includes relaxation of nuclear spins.11 See [108,109].12 See relaxation.13 rev[3]14 revGB-revPOC1516 chemical shift (NMR)17 18 Variation of the resonance frequency of a nucleus in nuclear magnetic resonance 19 (NMR) spectrometry as a consequence of its environment.20 Note 1: The chemical shift of a nucleus X, X, expressed as its frequency, X, relative 21 to that of a standard, ref, and defined as2223 X = (X – ref)/ref

2425 For 1H and 13C NMR the reference signal is usually that of tetramethylsilane (SiMe4). 26 Note 2: Chemical shift is usually reported in "parts per million" or ppm, where the 27 numerator has unit Hz, and the denominator has unit MHz, like the spectrometer's 28 operating frequency.29 Note 3: For historical reasons that predate Fourier-transform NMR, if a resonance 30 signal occurs at higher frequency than a reference signal, it is said to be downfield, and 31 if resonance occurs at lower frequency, the signal is upfield. Resonances downfield 32 from SiMe4 have positive -values, and resonances upfield from SiMe4 have negative 33 -values. These terms have been superseded, and deshielded and shielded are 34 preferred for downfield and upfield, respectively.35 See [110].36 See also shielding.37 rev[3]38 GB-revPOC3940 chemical species

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1 Ensemble of chemically identical molecular entities that can explore the same set of 2 molecular energy levels on the time scale of an experiment. The term is applied equally 3 to a set of chemically identical atomic or molecular structural units in a solid array.4 Note 1: For example, conformational isomers may be interconverted sufficiently 5 slowly to be detectable by separate NMR spectra and hence to be considered to be 6 separate chemical species on a time scale governed by the radiofrequency of the 7 spectrometer used. On the other hand, in a slow chemical reaction the same mixture of 8 conformers may behave as a single chemical species, i.e., there is virtually complete 9 equilibrium population of the total set of molecular energy levels belonging to the two

10 conformers.11 Note 2: Except where the context requires otherwise, the term is taken to refer to a 12 set of molecular entities containing isotopes in their natural abundance.13 Note 3: The definition given is intended to embrace not only cases such as graphite 14 and sodium chloride but also a surface oxide, where the basic structural units may not 15 be capable of isolated existence.16 Note 4: In common chemical usage, and in this Glossary, generic and specific 17 chemical names (such as radical or hydroxide ion) or chemical formulae refer either to 18 a chemical species or to a molecular entity.19 GB[3]2021 chemically induced dynamic nuclear polarization (CIDNP)22 Non-Boltzmann nuclear spin-state distribution produced in thermal or photochemical 23 reactions, usually from colligation and diffusion or disproportionation of radical pairs, 24 and detected by NMR spectroscopy as enhanced absorption or emission signals.25 See [111,112].26 revGB[3]2728 chemiexcitation29 Generation, by a chemical reaction, of an electronically excited molecular entity from 30 reactants in their ground electronic states.31 See [9].32 [3]3334 chemoselectivity35 Feature of a chemical reagent that reacts preferentially with one of two or more different 36 functional groups.37 Note 1: A reagent has a high chemoselectivity if reaction occurs with only a limited 38 number of different functional groups. For example, sodium tetrahydridoborate (NaBH4) 39 is a more chemoselective reducing agent than is lithium tetrahydridoaluminate (LiAlH4). 40 The concept has not been defined in more quantitative terms.

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1 Note 2: The term is also applied to reacting molecules or intermediates that exhibit 2 selectivity towards chemically different reagents.3 Note 3: Usage of the term chemospecificity for 100 % chemoselectivity is 4 discouraged.5 See [113].6 See also regioselectivity, stereoselectivity, stereospecificity.7 GB[3]89 chemospecificity

10 obsolete11 See chemoselectivity.12 GB[3]1314 chirality15 Property of a structure that is not superimposable on its mirror image.16 See [11,46].17 GB[3]1819 chirality centre20 chiral centre (superseded)21 Atom with attached groups such that the arrangement is not superimposable on its 22 mirror image.23 Note: Often this is a tetrahedral atom with four different groups attached, such as 24 CHBrClF, or C2 of CH3CHBrCH2CH3, or the sulfur of CH3S(=O)Ph, where the lone pair 25 is considered as a fourth group.26 See [11].27 See also stereogenic centre.28 GB[3]2930 chiral feature31 Structural characteristic rendering a molecule chiral.32 Examples: four different substituents on a carbon atom (chirality centre), 33 conformational helix, chiral axis (as in allenes XCH=C=CHX).3435 chiral recognition36 Attraction between molecules through noncovalent interactions that exhibit 37 complementarity only between partners with specific chirality.38 See also molecular recognition.3940 chromophore

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1 Part (atom or group of atoms) of a molecular entity in which the electronic transition 2 responsible for a given spectral band is approximately localized.3 Note 1: The term arose originally to refer to the groupings that are responsible for a 4 dye's colour.5 Note 2: The electronic transition can often be assigned as involving n, , *, , and/or 6 * orbitals whose energy difference falls within the range of the visible or UV spectrum.7 Note 3: The term has been extended to vibrational transitions in the infrared.8 See [9,114,115].9 GB[3]

1011 CIDNP12 Acronym for chemically induced dynamic nuclear polarization.13 GB[3]1415 cine-substitution16 Substitution reaction (generally aromatic) in which the entering group takes up a 17 position adjacent to that occupied by the leaving group.18 Example:19

202122 See also tele-substitution. 23 GB[3]2425 classical carbocation26 Carbocation whose electronic structure can be adequately described by two-electron-27 two-centre bonds, i.e., synonymous with carbenium ion.2829 clathrate30 See host, inclusion compound.31 GB[3]3233 coarctate34 Feature of a concerted transformation in which the primary changes in bonding occur 35 within a cyclic array of atoms but in which two nonbonding atomic orbitals on an atom 36 interchange roles with two bonding orbitals.

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1 Example: epoxidation with dimethydioxirane2

34 Note: Because the atomic orbitals that interchange roles are orthogonal, such a 5 reaction does not proceed through a fully conjugated transition state and is thus not a 6 pericyclic reaction. It is therefore not governed by the rules that express orbital 7 symmetry restrictions applicable to pericyclic reactions.8 See [116].9 See also pseudopericyclic.

1011 colligation12 Formation of a covalent bond by combination or recombination of two radicals.13 Note: This is the reverse of unimolecular homolysis.14 Example:15 HO· + H3C· → CH3OH16 GB[3]1718 collision complex19 Ensemble formed by two reaction partners, where the distance between them is equal 20 to the sum of the van der Waals radii of neighbouring atoms.21 See also encounter complex.22 [3]GB2324 common-ion effect (on rates)25 Reduction in the rate of certain reactions of a substrate RX in solution [by a path that 26 involves a pre-equilibrium with formation of R+ (or R–) ions as reaction intermediates] 27 caused by the addition to the reaction mixture of an electrolyte solute containing the 28 "common ion" X– (or X+).29 Example: the rate of solvolysis of chlorodiphenylmethane in acetone-water is 30 reduced by the addition of salts of the common ion Cl–, which causes a decrease in the 31 steady-state concentration of the diphenylmethyl cation:3233 Ph2CHCl Ph2CH+ + Cl– (free ions, not ion pairs)⇄34 Ph2CH+ + 2 OH2 → Ph2CHOH + H3O+

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12 Note: This retardation due to a common ion should be distinguished from the 3 acceleration due to a salt effect of all ions.4 rev[3]5 revGB-revPOC67 compensation effect8 Observation that a plot of T∆rS vs. ∆rH (frequently T∆‡S vs. ‡H) for a series of reactions 9 with a range of different substituents (or other unique variable such as solvent or

10 dissolved salt), are straight lines of approximately unit slope, so that, e.g., the terms 11 ∆‡H and T∆‡S partially compensate, and ∆‡G = ∆‡H – T∆‡S shows less variation than 12 ∆‡H or T∆‡S separately.13 Note: Frequently such ∆‡S vs. ∆‡H correlations are statistical artifacts, arising if 14 entropy and enthalpy are extracted from the variation of an equilibrium constant or a 15 rate constant with temperature, so that the slope and intercept of the van't Hoff plots 16 are correlated. The problem is avoided if the equilibrium constant and the enthalpy are 17 measured independently, for example by spectrophotometry and calorimetry, 18 respectively.19 See [117,118,119,120].20 See also isoequilibrium relationship, isokinetic relationship.21 rev[3]22 revGB-revPOC2324 complex25 Molecular entity formed by loose association involving two or more component 26 molecular entities (ionic or uncharged), or the corresponding chemical species. The 27 attraction between the components is often due to hydrogen-bonding or van der Waals 28 attraction and is normally weaker than a covalent bond.29 Note 1: The term has also been used with a variety of meanings in different contexts: 30 it is therefore best avoided when a more explicit alternative is applicable, such as adduct 31 when the association is a consequence of bond formation. 32 Note 2: In inorganic chemistry the term "coordination entity" is recommended instead 33 of "complex" [29,121].34 See also activated complex, adduct, charge transfer complex, electron-donor-35 acceptor complex, encounter complex, inclusion complex, -adduct, -adduct, 36 transition state.37 GB[3]3839 composite reaction

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1 Chemical reaction for which the expression for the rate of disappearance of a reactant 2 (or rate of appearance of a product) involves rate constants of more than a single 3 elementary reaction.4 Examples: "opposing reactions" (where rate constants of two opposed chemical 5 reactions are involved), "parallel reactions" (for which the rate of disappearance of any 6 reactant is governed by the rate constants relating to several simultaneous reactions 7 that form different products from a single set of reactants), and stepwise reactions.8 GB[3]9

10 comproportionation11 Any chemical reaction of the type A' + A" → 2 A 12 Example:13 Pb + PbO2 + 2 H2SO4 → 2 PbSO4 + 2 H2O ( in a lead battery)1415 Note: Other stoichiometries are possible, depending on the oxidation numbers of the 16 species.17 Reverse of disproportionation. The term "symproportionation" is also used.18 See [122].19 rev[3]20 revGB-revPOC2122 concerted23 Feature of a process in which two or more primitive changes occur within the same 24 elementary reaction. Such changes will normally be "energetically coupled".25 Note 1: The term "energetically coupled" means that the simultaneous progress of 26 the primitive changes involves a transition state of lower energy than that for their 27 successive occurrence.28 Note 2: In a concerted process the primitive changes may be synchronous or 29 asynchronous.30 See also bifunctional catalysis, potential-energy surface.31 GB[3]3233 condensation34 Reaction (usually stepwise) in which two or more reactants (or remote reactive sites 35 within the same molecular entity) yield a product with accompanying formation of water 36 or some other small molecule, e.g., ammonia, ethanol, acetic acid, hydrogen sulfide.37 Note 1: The mechanism of many condensation reactions has been shown to 38 comprise consecutive addition and elimination reactions, as in the base-catalysed 39 formation of (E)-but-2-enal (crotonaldehyde) from acetaldehyde, via dehydration of 3-

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1 hydroxybutanal (aldol). The overall reaction in this example is known as the aldol 2 condensation.3 Note 2: The term is sometimes also applied to cases where the formation of water 4 or other simple molecule does not occur, as in "benzoin condensation".5 [3]6 revGB78 condensed formula9 Linear representation of the structure of a molecular entity in which bonds are omitted.

10 Example: methyl 3-methylbutyl ether (isoamyl methyl ether, 11 (CH3)2CHCH2CH2OCH3, sometimes condensed further to (CH3)2CH[CH2]2OCH3)12 Note: This term is sometimes also called a line formula, because it can be written on 13 a single line, but the line formula explicitly shows all bonds.1415 configuration (electronic)16 Distribution of the electrons of an atom or a molecular entity over a set of one-electron 17 wavefunctions called orbitals, according to the Pauli principle.18 Note: From one configuration several states with different multiplicities may result. 19 For example, the ground electronic configuration of the oxygen molecule (O2) is20 1g

2, 1u2, 2g

2, 2u2, 1u

4, 3g2, 1g

2

21 resulting in22 3g

–, 1g, and 1g+ multiplets

23 See [8,9].24 GB[3]2526 configuration (molecular)27 Arrangement in space of the atoms of a molecular entity that distinguishes it from any 28 other molecular entity having the same molecular formula and connectivity and that is 29 not due to conformational differences (rotation about single bonds).30 See [11].31 [3]3233 conformations34 Different spatial arrangements of a molecular entity that can be interconverted by 35 rotation about one or more formally single bonds. 36 Note 1: Different conformations are often not considered to be stereoisomeric, 37 because interconversion is rapid. 38 Note 2: Different or equivalent spatial arrangements of ligands about a central atom, 39 such as those interconverted by pyramidal inversion (of amines) or Berry

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1 pseudorotation (as of PF5) and other "polytopal rearrangements", are sometimes 2 considered conformations, but they are properly described as configurations.3 See [11].4 rev[3]5 revGB67 conformational isomers8 conformers9

10 conformer11 Conformation of a molecular entity that corresponds to a minimum on the potential-12 energy surface of that molecular entity.13 Note: The distinction between conformers and isomers is the height of the barrier for 14 interconversion. Isomers are stable on macroscopic timescales because the barrier for 15 interconversion is high, whereas a conformer cannot persist on a macroscopic 16 timescale because the interconversion between conformations is achieved rapidly.17 See [11].181920 conjugate acid21 Brønsted acid BH+ formed on protonation (hydronation) of the base B.22 Note 1: B is called the conjugate base of the acid BH+.23 Note 2: The conjugate acid always carries one unit of positive charge more than the 24 base, but the absolute charges of the species are immaterial to the definition. For 25 example: the Brønsted acid HCl and its conjugate base Cl– constitute a conjugate acid-26 base pair, and so do NH4

+ and its conjugate base NH3.27 revGB[3]2829 conjugated system, conjugation30 Molecular entity whose structure may be represented as a system of alternating multiple 31 and single (or ) bonds: e.g.,32 CH2=CH–CH=CH2 or CH2=CH–CΞN33 Note: In such systems, conjugation is the interaction of one p-orbital with another p-34 orbital (or d-orbital) across an intervening bond, including the analogous interaction 35 involving a p-orbital containing an unshared electron pair, e.g.,

3637 or the interaction across a double bond whose system does not interact with the p 38 orbitals of the conjugated system, as in39 CH2=C=C=CH2

40 See also cross-conjugation, delocalization, resonance, through-conjugation.

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1 GB[3]23 connectivity4 Description of which atoms are bonded to which other atoms.5 Note: Connectivity is often displayed in a line formula or other structure showing 6 which atoms are bonded to which other atoms, but with minimal or no indication of bond 7 multiplicity.8 Example: The connectivity of propyne is specified by CH3CCH.9 revGB[3]

1011 conrotatory12 Stereochemical feature of an electrocyclic reaction in which the substituents at the 13 interacting termini of the conjugated system rotate in the same sense (both clockwise 14 or both counterclockwise).15 See also disrotatory.16 rev[3]17 revGB-revPOC1819 conservation of orbital symmetry20 An approach to understanding pericyclic reactions that focuses on a symmetry 21 element (e.g., a reflection plane) that is retained along a reaction pathway. If each of 22 the singly or doubly occupied orbitals of the reactant(s) is of the same symmetry as a 23 similarly occupied orbital of the product(s), that pathway is "allowed" by orbital 24 symmetry conservation. If instead a singly or doubly occupied orbital of the reactant(s) 25 is of the same symmetry as an unoccupied orbital of the product(s), and an unoccupied 26 orbital of the reactant(s) is of the same symmetry as a singly or doubly occupied orbital 27 of the product(s), that pathway is "forbidden" by orbital symmetry conservation.28 Note 1: This principle permits the qualitative construction of correlation diagrams to 29 show how molecular orbitals transform and how their energies change during chemical 30 reactions.31 Note 2: Considerations of orbital symmetry are frequently grossly simplified in that, 32 for example, the and * orbitals of a carbonyl group in an asymmetric molecule are 33 treated as having the same topology (pattern of local nodal planes) as those of a 34 symmetric molecule (e.g., CH2=O), despite the absence of formal symmetry elements.35 See [107,123].36 See also orbital symmetry.3738 constitutional isomers39 Species (or molecular entities) with the same atomic composition (molecular formula) 40 but with different connectivity.

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1 Note: The term structural isomers is discouraged, because all isomers differ in 2 structure and because isomers may be constitutional, configurational, or 3 conformational.4 See [11].56 contributing structure7 resonance form8 rev[3]9 revGB-revPOC

1011 coordinate covalent bond12 dative bond1314 coordination15 Formation of a covalent bond, the two shared electrons of which come from only one 16 of the two partners linked by the bond, as in the reaction of a Lewis acid and a Lewis 17 base to form a Lewis adduct; alternatively, the bonding formed in this way.18 Note: In the former sense, it is the reverse of unimolecular heterolysis. 19 See also dative bond, -adduct.20 rev[3]21 revGB2223 coordination number24 Number of other atoms directly linked to a specified atom in a chemical species 25 regardless of the number of electrons in the bonds linking them [29, Rule IR-10.2.5]. 26 For example, the coordination number of carbon in methane or of phosphorus in 27 triphenylphosphane oxide (triphenylphosphine oxide) is four whereas the coordination 28 number of phosphor is five in phosphorus pentafluoride.29 Note: The term is used in a different sense in the crystallographic description of ionic 30 crystals.31 revGB[3]3233 coronate34 See crown ether.35 revGB[3]3637 correlation analysis38 Use of empirical correlations relating one body of experimental data to another, with the 39 objective of finding quantitative relationships among the factors underlying the 40 phenomena involved. Correlation analysis in organic chemistry often uses linear Gibbs-

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1 energy relations (formerly linear free-energy relation, LFER) for rates or equilibria of 2 reactions, but the term also embraces similar analysis of physical (most commonly 3 spectroscopic) properties and of biological activity.4 See [124,125,126,127,128].5 See also linear free-energy relation, quantitative structure-activity relationship 6 (QSAR).7 GB[3]89 coupling constant (spin-spin coupling constant)

10 J11 (Unit: Hz)12 Quantitative measure of nuclear spin-spin coupling in nuclear magnetic resonance 13 spectroscopy.14 Note: Spin-spin coupling constants have been correlated with atomic hybridization 15 and with molecular conformations.16 See [129,130,131].17 revGB[3]1819 covalent bond20 Stabilizing interaction associated with the sharing of electron pairs between two atomic 21 centres of a molecular entity, leading to a characteristic internuclear distance.22 See also agostic, coordination, hydrogen bond, multi-centre bond.23 See [8].24 revGB[3]2526 Cox–Yates equation27 Generalization of the Bunnett-Olsen equation of the form2829 lg([SH+])/[S]) – lg{[H+]} = m*X + pKSH+

3031 where [H+] is the amount concentration of acid, X is the activity-coefficient ratio 32 lg({s{H+}/{SH+}) for an arbitrary reference base, pKSH+ is the thermodynamic 33 dissociation constant of SH+, and m* is an empirical parameter derived from linear 34 regression of the left-hand side vs. X. Arguments in the lg functions should be unitless. 35 Thus, the reduced quantities should be used: {[H+]} is [H+]/units.36 Note: The function X is called excess acidity because it gives a measure of the 37 difference between the acidity of a solution and that of an ideal solution of the same 38 concentration. In practice X = – (Ho + lg{[H+]}) and m* = 1 – , where Ho is the Hammett 39 acidity function and is the slope in the Bunnett-Olsen equation.40 See [132,133,134].

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1 See Bunnett-Olsen equation.2 rev[3]3 revGB45 critical micellisation concentration (cmc) 6 critical micelle concentration7 Relatively small range of concentrations separating the limit below which virtually no 8 micelles are detected and the limit above which virtually all additional surfactant 9 molecules form micelles.

10 Note 1: Many physical properties of surfactant solutions, such as conductivity or light 11 scattering, show an abrupt change at a particular concentration of the surfactant, which 12 can be taken as the cmc.13 Note 2: As values obtained using different properties are not quite identical, the 14 method by which the cmc is determined should be clearly stated.15 See [135].16 See also inverted micelle.17 rev[3]18 revGB-revPOC1920 cross-conjugation21 Phenomenon of three conjugated groups, two pairs of which exhibit conjugation but 22 without through-conjugation of all three, as in 2-phenylallyl, benzoate anion, divinyl 23 ether, or m-xylylene [1,3-C6H4(CH2)2].24 See [64].25 rev[3]26 revGB-revPOC2728 crown ether29 Molecular entity comprising a monocyclic ligand assembly that contains three or more 30 binding sites held together by covalent bonds and capable of binding a guest in a central 31 (or nearly central) position. The adducts formed are sometimes known as "coronates". 32 The best known members of this group are macrocyclic polyethers, such as 18-crown-33 6, containing several repeating units –CR2–CR2O– (where R is most commonly H).

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12 See [136,137].3 See also host.4 rev[3]5 revPOC67 cryptand8 Molecular entity comprising a cyclic or polycyclic assembly of binding sites that contains 9 three or more binding sites held together by covalent bonds, and which defines a

10 molecular cavity in such a way as to bind (and thus "hide" in the cavity) another 11 molecular entity, the guest (a cation, an anion or a neutral species), more strongly than 12 do the separate parts of the assembly (at the same total concentration of binding sites).13 Note 1: The adduct thus formed is called a "cryptate". The term is usually restricted 14 to bicyclic or oligocyclic molecular entities.15 Example:

1617 Note 2: Corresponding monocyclic ligand assemblies (crown ether) are sometimes 18 included in this group, if they can be considered to define a cavity in which a guest can 19 hide. The terms "podand" and "spherand" are used for certain specific ligand 20 assemblies. Coplanar cyclic polydentate ligands, such as porphyrins, are not normally 21 regarded as cryptands.22 See [138].23 See also host. See also [139].24 GB[3]2526 curly arrows27 Symbols for depicting the flow of electrons in a chemical reaction or to generate 28 additional resonance forms.29 Note 1: The tail of the curly arrow shows where an electron pair originates, and the 30 head of the curly arrow shows where the electron pair goes.31 Note 2: Single-headed curly arrows are used to depict the flow of unpaired electrons.32 Examples:33

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123 See electron pushing.45 Curtin-Hammett principle6 Statement that in a chemical reaction that yields one product (X) from one isomer (A') 7 and a different product (Y) from another isomer (A") (and provided these two isomers 8 are rapidly interconvertible relative to the rate of product formation, whereas the 9 products do not undergo interconversion) the product composition is not directly related

10 to the relative concentrations of the isomers in the substrate; it is controlled only by the 11 difference in standard Gibbs energies (G‡

A' – G‡A") of the respective transition states.

12 Note 1: The product composition is given by [Y]/[X] = (kYk1)/(k–1kX) or KckY/kX, where 13 Kc is the equilibrium constant, [A"]/[A'], and where kY and kX are the respective rate 14 constants of their reactions; these parameters are usually unknown.15 Note 2: The energy diagram below represents the transformation of rapidly 16 interconverting isomers A' and A" into products X and Y.17

1819

2021

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1 Note 3: A related concept is the Winstein-Holness equation for the overall rate of 2 product formation in a system of rapidly interconverting isomers A' and A". The overall 3 rate constant k is given by45 k = x'kX + x"kY

67 where x' and x" are the respective mole fractions of the isomers at equilibrium [15]. 8 However, since x' and x" are simple functions of Kc, the apparent dependence of the 9 overall rate on the relative concentrations of A' and A" disappears. The overall rate

10 constant is the sum kX + KckY, which is a function of the same quantities that determine 11 the product ratio.12 See [140]. See also [15,141,142].13 revGB[3]1415 cybotactic region16 That part of a solution in the vicinity of a solute molecule in which the ordering of the 17 solvent molecules is modified by the presence of the solute molecule. The term solvent 18 "cosphere" of the solute has also been used.19 See [143,144].20 GB[3]2122 cyclization23 Formation of a ring compound from a chain by formation of a new bond.24 See also annulation.25 GB[3]2627 cycloaddition28 Reaction in which two or more unsaturated molecules (or parts of the same molecule) 29 combine with the formation of a cyclic adduct in which there is a net reduction of the 30 bond multiplicity.31 Note 1: The following two systems of notation have been used for the more detailed 32 specification of cycloadditions, of which the second, more recent system (described 33 under (2)) is preferred:34 (1) An (i+j+...) cycloaddition is a reaction in which two or more molecules (or parts 35 of the same molecule), respectively, provide units of i, j,... linearly connected 36 atoms: these units become joined at their respective termini by new σ bonds so 37 as to form a cycle containing (i+j+...) atoms. In this notation, (a) a Diels-Alder 38 reaction is a (4+2) cycloaddition, (b) the initial reaction of ozone with an alkene is 39 a (3+2) cycloaddition, and (c) the reaction of norbornadiene below is a (2+2+2)

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1 cycloaddition. (Parentheses are used to indicate the numbers of atoms, but 2 brackets are also used.)

345 (2) The symbolism [i+j+...] for a cycloaddition identifies the numbers i, j,... of 6 electrons in the interacting units that participate in the transformation of reactants 7 to products. In this notation the reaction (a) and (b) of the preceding paragraph 8 would both be described as [2+4] cycloadditions, and (c) as a [2+2+2] 9 cycloaddition. The symbol a or s (a = antarafacial, s = suprafacial) is often added

10 (usually as a subscript after the number to designate the stereochemistry of 11 addition to each fragment. A subscript specifying the orbitals, viz., , (with their 12 usual significance) or n (for an orbital associated with a single atom), may be 13 added as a subscript before the number. Thus the normal Diels-Alder reaction is 14 a [4s+2s] or [4s + 2s] cycloaddition, whilst the reaction15

16

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1 would be a [14a+2s] or [14a + 2s] cycloaddition, leading to the stereoisomer shown, 2 with hydrogens anti. (Brackets are used to indicate the numbers of electrons, and 3 they are also used instead of parentheses to denote the numbers of atoms.)4 Note 2: Cycloadditions may be pericyclic reactions or (non-concerted) stepwise 5 reactions. The term "dipolar cycloaddition" is used for cycloadditions of 1,3-dipolar 6 compounds.7 See [107,145,146]. 8 See also cheletropic reaction, ene reaction, pericyclic reaction.9 revGB[3]

1011 cycloelimination12 Reverse of cycloaddition. The term is preferred to the synonyms "cycloreversion", 13 "retro-addition", and "retrocycloaddition".14 GB[3]1516 cycloreversion17 obsolete18 See cycloelimination.19 GB[3]2021 dative bond22 obsolescent23 Coordination bond formed between two chemical species, one of which serves as a 24 donor and the other as an acceptor of the electron pair that is shared in the bond.25 Examples: the N–B bond in H3N+–B–H3, the S–O bond in (CH3)2S+–O–.26 Note 1: A distinctive feature of dative bonds is that their minimum-energy rupture in 27 the gas phase or in inert solvent follows the heterolytic bond-cleavage path.28 Note 2: The term is obsolescent because the distinction between dative bonds and 29 ordinary covalent bonds is not useful, in that the precursors of the bond are irrelevant: 30 H3N+–B–H3 is the same molecule, with the same bonds, regardless of whether the 31 precursors are considered to have been H3N + BH3 or H3N·+ + –BH3.32 See [8].33 See coordination.34 rev[3]35 revGB3637 degenerate chemical reaction38 See identity reaction.39 GB[3]40

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1 degenerate rearrangement2 Chemical reaction in which the product is indistinguishable (in the absence of isotopic 3 labelling) from the reactant.4 Note 1: The term includes both "degenerate intramolecular rearrangements" and 5 reactions that involve intermolecular transfer of atoms or groups ("degenerate 6 intermolecular reactions"): both are degenerate isomerizations.7 Note 2: The occurrence of degenerate rearrangements may be detectable by isotopic 8 labelling or by dynamic NMR techniques. For example: the [3,3]sigmatropic 9 rearrangement of hexa-1,5-diene (Cope rearrangement),

10 Note 3: Synonymous but less preferable terms are "automerization", "permutational 11 isomerism", "isodynamic transformation", "topomerization".12 See [147]. 13 See also fluxional, molecular rearrangement, narcissistic reaction, valence isomer.14 GB[3]1516 delocalization17 Quantum-mechanical concept most usually applied in organic chemistry to describe the 18 redistribution of electrons in a conjugated system, where each link has a fractional 19 double-bond character, or a non-integer bond order, rather than electrons that are

20 localized in double or triple bonds.21 Note 1: There is a corresponding "delocalization energy", identifiable with the 22 stabilization of the system relative to a hypothetical alternative in which formal 23 (localized) single and double bonds are present. Some degree of delocalization is 24 always present and can be estimated by quantum mechanical calculations. The effects 25 are particularly evident in aromatic systems and in symmetrical molecular entities in 26 which a lone pair of electrons or a vacant p-orbital is conjugated with a double bond 27 (e.g., carboxylate ions, nitro compounds, enamines, the allyl cation).28 Note 2: Delocalization in such species may be represented by partial bonds or by 29 resonance (here symbolized by a two-headed arrow) between resonance forms.30

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12 These examples also illustrate the concomitant delocalization of charge in ionic 3 conjugated systems. Analogously, delocalization of the spin of an unpaired electron 4 occurs in conjugated radicals.5 Note 3: Delocalization is not limited to electrons. Hyperconjugation is the 6 delocalization of electrons of bonds.7 GB[3]89 dendrimer

10 Large molecule constructed from a central core with repetitive branching and multiple 11 functional groups at the periphery. 12 See [148].13 Example: (with 48 CH2OH at the periphery) 14

151617 Note: The name comes from the Greek (dendron), which translates to "tree". 18 Synonymous terms include arborols and cascade molecules.19 See [148,149,150,151,152].2021 deshielding

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1 See shielding.2 GB[3]34 detachment5 Reverse of attachment.6 GB[3]789

10 detailed balancing11 Principle that when equilibrium is reached in a reaction system (containing an arbitrary 12 number of components and reaction paths), as many atoms, in their respective 13 molecular entities, will pass forward in a given finite time interval as will pass backward 14 along each individual path.15 Note 1: It then follows that the reaction path in the reverse direction must in every 16 detail be the reverse of the reaction path in the forward direction (provided that the 17 system is at equilibrium).18 Note 2: The principle of detailed balancing is a consequence for macroscopic 19 systems of the principle of microscopic reversibility.20 GB[3]2122 diamagnetism23 Property of substances having a negative magnetic susceptibility (χ), whereby they are 24 repelled out of a magnetic field.25 See also paramagnetism.26 [3]2728 diastereoisomerism29 Stereoisomerism other than enantiomerism.30 See diastereoisomers [11].31 GB[3]3233 diastereomeric excess (diastereoisomeric excess)34 x1 – x2, where x1 and x2 (with x1 + x2 = 1) are the mole fractions of two diastereoisomers 35 in a mixture, or the fractional yields of two diastereoisomers formed in a reaction.36 Note: Frequently this term is abbreviated to d.e.37 See stereoselectivity, diastereoisomers.38 See [11].3940 diastereomeric ratio

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1 x1/x2, where x1 and x2 are the mole fractions of two diastereoisomers in a mixture formed 2 in a reaction.3 Note: Frequently this term is abbreviated to d.r.4 See stereoselectivity, diastereoisomers.5 See [11].67 diastereoisomers (diastereomers)8 Stereoisomers not related as mirror images of each other.9 Note: Diastereoisomers are characterized by differences in physical properties, and

10 by differences in chemical behaviour toward chiral as well as achiral reagents.11 See [11].1213 diastereoselectivity14 Preferential formation in a chemical reaction of one diastereoisomer over another.15 Note: This can be expressed quantitatively by the diastereoisomeric excess or by the 16 diastereomeric ratio, which is preferable because it is more closely related to a Gibbs-17 energy difference.18 See [11].19 See selectivity.2021 dielectric constant22 obsolete23 See [12].24 See permittivity (relative).25 revGB[3]2627 dienophile28 Ene or yne component of a Diels-Alder reaction, including compounds with hetero-29 double bonds and hetero-triple bonds.30 See cycloaddition.31 rev[3]32 revGB-revPOC3334 diffusion-controlled rate35 See encounter-controlled rate, microscopic diffusion control. Contrast mixing control.36 GB[3]3738 dimerization39 Transformation of a molecular entity A to give a molecular entity A2.40 Examples:

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1 2 CH3· → CH3CH3

2 2 CH3COCH3 → (CH3)2C(OH)CH2COCH3

3 2 RCOOH → (RCOOH)2

4 See also association.5 GB[3]67 Dimroth-Reichardt ET parameter8 Quantitative measure of solvent polarity, based on the wavelength, max, of the longest-9 wavelength intramolecular charge-transfer absorption band of the solvatochromic

10 betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridin-1-ium-1-yl)phenolate.11

1213 See [17,153,154,155].14 See solvent parameter.15 rev[3]16 revGB-revPOC1718 dipolar aprotic solvent19 See dipolar non-HBD solvent.20 rev[3]21 revGB-revPOC2223 dipolar non-HBD solvent (Non-hydrogen-bond donating solvent)24 Solvent with a comparatively high relative permittivity ("dielectric constant"), greater 25 than ca. 15, and composed of molecules that have a sizable permanent dipole moment 26 and that, although it may contain hydrogen atoms, cannot donate suitably labile 27 hydrogen atoms to form strong solvent-solute hydrogen bonds.28 Examples: dimethyl sulfoxide, acetonitrile, acetone, as contrasted with methanol and 29 N-methylformamide.30 Note 1: The term “dipolar” refers to solvents whose molecules have a permanent 31 dipole moment, in contrast to solvents whose molecules have no permanent dipole 32 moment and should be termed “apolar” or "nonpolar".33 Note 2: Non-HBD solvents are often called aprotic, but this term is misleading 34 because a proton can be removed by a sufficiently strong base. The aprotic nature of 35 a solvent molecule means that its hydrogens are in only covalent C–H bonds and not

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1 in polar O–H+ or N–H+ bonds that can serve as hydrogen-bond donors. Use of 2 “aprotic” is therefore discouraged, unless the context makes the term unambiguous.3 Note 3: It is recommended to classify solvents according to their capability to donate 4 or not donate, as well as to accept or not accept, hydrogen bonds to or from the solute, 5 as follows:6 Hydrogen-bond donating solvents (short: HBD solvents), formerly protic solvents7 Non-hydrogen-bond donating solvents (short: non-HBD solvents), formerly aprotic 8 solvents9 Hydrogen-bond accepting solvents (short: HBA solvents)

10 Non-hydrogen-bond accepting solvents (short: non-HBA solvents)11 See [156,157].1213 dipole-dipole excitation transfer14 Förster resonance-energy transfer (FRET)15 See [9].1617 dipole-dipole interaction18 Intermolecular or intramolecular interaction between molecules or groups having a 19 permanent electric dipole moment. The strength of the interaction depends on the 20 distance and relative orientation of the dipoles.21 Note: A dipole/dipole interaction is a simplification of the electrostatic interactions 22 between molecules that originate from asymmetries in the electron densities. Such 23 interactions can be described more correctly by the use of higher-order multipole 24 moments.25 See also van der Waals forces.26 rev[3]27 revPOC2829 dipole-induced dipole forces30 See van der Waals forces.31 GB[3]3233 diradical34 biradical 35 Even-electron molecular entity with two (possibly delocalized) radical centres that act 36 nearly independently of each other, e.g.,

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12 Note 1: Species in which the two radical centres interact significantly are often 3 referred to as "diradicaloids". If the two radical centres are located on the same atom, 4 the species is more properly referred to by its generic name: carbene, nitrene, etc.5 Note 2: The lowest-energy triplet state of a diradical lies below or at most only a little 6 above its lowest singlet state (usually judged relative to kBT, the product of the 7 Boltzmann constant kB and the absolute temperature T). If the two radical centres 8 interact significantly, the singlet state may be more stable. The states of those diradicals 9 whose radical centres interact particularly weakly are most easily understood in terms

10 of a pair of local doublets.11 Note 3: Theoretical descriptions of low-energy states of diradicals display two 12 unsaturated valences: the dominant valence-bond structures have two dots, the low-13 energy molecular orbital configurations have only two electrons in two approximately 14 nonbonding molecular orbitals, and two of the natural orbitals have occupancies close 15 to one.16 See [9,158,159,160,161,162,163].17 See also carbene, nitrene.18 GB[3]1920 dispersion forces21 See London forces, van der Waals forces.22 GB[3]2324 disproportionation25 Any chemical reaction of the type A + A ⇋ A' + A", where A, A' and A" are different 26 chemical species. 27 Examples:28 2 R2COH· (ketyl radical) → R2C=O + R2CHOH29 Cannizzaro reaction: 2 ArCH=O → ArCH2OH + ArCOOH3031 Note 1: The reverse of disproportionation is called comproportionation.32 Note 2: A special case of disproportionation (or "dismutation") is "radical 33 disproportionation", exemplified by3435 ·CH2CH3 + ·CH2CH3 → CH2=CH2 + CH3CH3

3637 Note 3: A somewhat more restricted usage of the term prevails in inorganic 38 chemistry, where A, A' and A" are of different oxidation states.39 GB[3]40

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1 disrotatory2 Stereochemical feature of an electrocyclic reaction in which the substituents at the 3 interacting termini of the conjugated system rotate in opposite senses (one clockwise 4 and the other counterclockwise).5 See also conrotatory.6 rev[3]7 revGB-revPOC89 dissociation

10 Separation of a molecular entity into two or more molecular entities (or any similar 11 separation within a polyatomic molecular entity).12 Example:13 NH4

+(aq) → H3O+ + NH3 or CH3COOH(aq) → H3O+ + CH3CO2

–

14 Note 1: Although the separation of the constituents of an ion pair into free ions is a 15 dissociation, the ionization that produces the ion pair is not a dissociation, because the 16 ion pair is a single molecular entity.17 Note 2: The reverse of dissociation is association.18 rev[3]19 revGB-revPOC2021 distonic ion 22 Radical ion in which charge and radical sites are separated.23 Example: •CH2CH2OCH2

+

24 See [164,165].25 rev[3]26 revGB-revPOC2728 distortion interaction model (Activation Strain Model)29 Method for analyzing activation energy as the sum of the energies to distort the 30 reactants into the geometries they have in transition states plus the energy of interaction 31 between the two distorted reactants.32 See [166].3334 distribution ratio35 partition ratio36 Ratio of concentrations of a solute in a mixture of two immiscible phases at equilibrium.37 See also Hansch constant.38 See [167].3940 di--methane rearrangement

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1 Photochemical reaction of a molecular entity with two -systems separated by a 2 saturated carbon, to form an ene-substituted cyclopropane.3 Pattern:

45 See [9,168].67 donicity (also called donor number, DN, which is a misnomer)8 Quantitative experimental measure of the Lewis basicity of a molecule B, expressed as 9 the negative enthalpy of the formation of the 1:1 complex B·SbCl5.

10 Note: donicity is often used as a measure of a solvent’s basicity.11 See also acceptor parameter, Lewis basicity.12 See [63,169,170].1314 downfield 15 superseded but still widely used in NMR to mean deshielded.16 See chemical shift, shielding.17 rev[3]18 revGB1920 driving force21 (1) Negative of the Gibbs energy change (∆rG0) on going from the reactants to the 22 products of a chemical reaction under standard conditions. Also called affinity.23 (2) Qualitative term that relates the favorable thermodynamics of a reaction to a specific 24 feature of molecular structure, such as the conversion of weaker bonds into stronger 25 (CH3–H + Br–Br → CH3–Br + H–Br), neutralization of an acid (Claisen condensation of 26 2 CH3COOEt + EtO– to CH3COCH–COOEt + EtOH), or increase of entropy 27 (cycloelimination of cyclohexene to butadiene + ethylene).28 Note 1: This term is a misnomer, because favorable thermodynamics is due to 29 energy, not force.30 Note 2: This term has also been used in connection with photoinduced electron 31 transfer reactions, to indicate the negative of the estimated standard Gibbs energy 32 change for the outer sphere electron transfer (∆ETG0) [9].33 rev[3]34 GB3536 dual substituent-parameter equation37 Any equation that expresses substituent effects in terms of two parameters.

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1 Note: In practice the term is used specifically for an equation for modeling the effects 2 of meta- and para-substituents X on chemical reactivity, spectroscopic properties, etc. 3 of a probe site in benzene or other aromatic system.4 PX = I I + R R

5 where PX is the magnitude of the property for substituent X, expressed relative to the 6 property for X = H; I and R are inductive (or polar) and resonance substituent 7 constants, respectively, there being various scales for R; I and R are the 8 corresponding regression coefficients.9 See [171,172,173].

10 See also extended Hammett equation, Yukawa-Tsuno equation.11 GB[3]1213 dynamic NMR14 NMR spectroscopy of samples undergoing chemical reactions.15 Note: Customarily this does not apply to samples where the composition of the 16 sample, and thus its NMR spectrum, changes with time, but rather to samples at 17 equilibrium, without any net reaction. The occurrence of chemical reactions is 18 manifested by features of the NMR line-shape or by magnetization transfer.19 See chemical flux.2021 dyotropic rearrangement22 Process in which two bonds simultaneously migrate intramolecularly.23 Example

2425 See [174,175,176].26 rev[3]27 revPOC2829 educt30 deprecated: usage strongly discouraged31 starting material, reactant32 Note: This term should be avoided and replaced by reactant, because it means 33 "something that comes out", not "something that goes in".34 revGB[3]3536 effective charge, Zeff

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1 Net positive charge experienced by an electron in a polyelectronic atom, which is less 2 than the full nuclear charge because of shielding by the other electrons.3 rev[3]4 revGB-revPOC56 effective molarity (effective concentration)7 Ratio of the first-order rate constant or equilibrium constant of an intramolecular 8 reaction involving two functional groups within the same molecular entity to the second-9 order rate constant or equilibrium constant of an analogous intermolecular elementary

10 reaction.11 Note: This ratio has unit of concentration, mol dm–3 or mol L–1, sometimes denoted 12 by M.13 See [177].14 See also intramolecular catalysis.15 GB[3]161718 eighteen-electron rule19 Electron-counting rule that the number of nonbonding electrons at a metal plus the 20 number of electrons in the metal-ligand bonds should be 18.21 Note: The 18-electron rule in transition-metal chemistry is an analogue of the Lewis 22 octet rule.23 [3]24 revGB-revPOC2526 electrocyclic reaction (electrocyclization)27 Molecular rearrangement that involves the formation of a bond between the termini 28 of a fully conjugated linear -electron system (or a linear fragment of a -electron 29 system) and a decrease by one in the number of π bonds, or the reverse of that process.30 Examples:

3132 Note: The stereochemistry of such a process is termed "conrotatory" if the 33 substituents at the interacting termini of the conjugated system both rotate in the same 34 sense, as in

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12 or "disrotatory" if one terminus rotates in a clockwise and the other in a 3 counterclockwise sense, as in

45 See also pericyclic reaction.6 [3]7 revGB-revPOC89 electrofuge

10 Leaving group that does not carry away the bonding electron pair.11 Examples: In the nitration of benzene by NO2

+, H+ is the electrofuge, and in an SN1 12 reaction the carbocation is the electrofuge.13 Note 1: Electrofugality characterizes the relative rates of atoms or groups to depart 14 without the bonding electron pair. Electrofugality depends on the nature of the reference 15 reaction and is not the reverse of electrophilicity [178].16 Note 2: For electrofuges in SN1 reactions see [179].17 See also electrophile, nucleofuge.18 rev[3]19 revGB-revPOC2021 electromeric effect22 obsolete23 rev[3]24 revGB-revPOC2526 electron acceptor27 Molecular entity to which an electron may be transferred.28 Examples: 1,4-dinitrobenzene,1,1'-dimethyl-4,4'-bipyridinium dication, 29 benzophenone.30 Note 1: A group that accepts electron density from another group is not called an 31 electron acceptor but an electron-withdrawing group.32 Note 2: A Lewis acid is not called an electron acceptor but an electron-pair acceptor.33 revGB[3]3435 electron affinity36 Energy released when an additional electron (without excess energy) attaches itself to 37 a molecular entity (often electrically neutral but not necessarily).

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1 Note 1: Equivalent to the minimum energy required to detach an electron from a 2 singly charged negative ion.3 Note 2: Measurement of electron affinities is possible only in the gas phase, but there 4 are indirect methods for evaluating them from solution data, such as polarographic half-5 wave potentials or charge-transfer spectra.6 See [8,180,181].7 rev[3]8 revGB9

10 electron capture11 Transfer of an electron to a molecular entity, resulting in a molecular entity of 12 (algebraically) increased negative charge.13 revGB1415 electron-deficient bond16 Bond between adjacent atoms that is formed by fewer than two electrons.17 Example:

1819 Note: The B–H–B bonds are also called "two-electron three-centre bonds".20 GB[3]2122 electron density23 The electron density at a point with coordinates x,y,z in an atom or molecular entity is 24 the product of the probability P(x,y,z) (units: m-3) of finding an electron at that point with 25 the volume element dx dy dz (units: m3).26 Note: For many purposes (e.g., X-ray scattering, forces on atoms) the system 27 behaves as if the electrons were spread out into a continuous distribution, which is a 28 manifestation of the wave-particle duality.29 See also atomic charge, charge density.30 rev[3]3132 electron donor33 Molecular entity that can transfer an electron to another molecular entity, or to the 34 corresponding chemical species.35 Note 1: After the electron transfer the two entities may separate or remain 36 associated.

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1 Note 2: A group that donates electron density to another group is an electron-2 donating group, regardless of whether the donation is of or electrons.3 Note 3: A Lewis base is not called an electron donor, but an electron-pair donor.4 See also electron acceptor.5 rev[3]6 revGB-revPOC78 electron-donor-acceptor complex 9 obsolete

10 charge-transfer complex.11 See also adduct, coordination.1213 electron-pair acceptor14 Lewis acid.15 GB[3]16171819 electron-pair donor20 Lewis base.21 GB[3]2223 electron pushing24 Method using curly arrows for showing the formal movement of an electron pair (from 25 a lone pair or a or bond) or of an unpaired electron, in order to generate additional 26 resonance forms or to denote a chemical reaction.27 Note 1: The electron movement may be intramolecular or intermolecular.28 Note 2: When a single electron is transferred, a single-headed curly arrow or “fish-29 hook” is used, but the electron movement in the opposite direction is redundant and 30 sometimes omitted.31 Examples:

323334 electron transfer35 Transfer of an electron from one molecular entity to another, or between two localized 36 sites in the same molecular entity.

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1 See also inner sphere (electron transfer), Marcus equation, outer sphere (electron 2 transfer).3 GB[3]45 electron-transfer catalysis6 Process describing a sequence of reactions such as shown in equations (1)-(3), leading 7 from A to B, via species A·– and B·– that have an extra electron:8 A + e– → A·– (1)9 A·– → B·– (2)

10 B·– + A → B + A·– (3)11 Note 1: An analogous sequence involving radical cations (A·+, B·+) also occurs.12 Note 2: The most notable example of electron-transfer catalysis is the SRN1 (or 13 T+DN+AN) reaction of haloarenes with nucleophiles.14 Note 3: The term has its origin in an analogy to acid-base catalysis, with the electron 15 instead of the proton. However, there is a difference between the two catalytic 16 mechanisms, since the electron is not a true catalyst, but rather behaves as the initiator 17 of a chain reaction. "Electron-transfer induced chain reaction" is a more appropriate 18 term for the mechanism described by equations (1)-(3).19 See [182,183].20 GB[3]2122 electronation23 obsolete24 See reduction.25 rev[3]26 revPOC2728 electronegativity29 Measure of the power of an atom to attract electrons to itself.30 Note 1: The concept has been quantified by a number of authors. The first, due to 31 Pauling, is based on bond-dissociation energies, Ed (units: eV), and is anchored by 32 assigning the electronegativity of hydrogen as r,H = 2.1. For atoms A and B3334 r,A – r,B = (eV)-1/2 Ed(A–B)/eV – ½ [Ed(A–A) + Ed(B–B)]/eV}3536 where r denotes the Pauling relative electronegativity; it is relative in the senses that 37 the values are of dimensionless dimension 1 and that only electronegativity differences 38 are defined on this empirical scale. See [12] section 2.5.

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1 Note 2: Alternatively, the electronegativity of an element, according to the Mulliken 2 scale, is the average of its atomic ionization energy and electron affinity. Other scales 3 have been developed by Allred and Rochow, by Sanderson, and by Allen.4 Note 3: Absolute electronegativity is property derived from density functional theory 5 [8].6 See [75,184,185,186,187,188,189,190].7 rev[3]8 revGB9

10 electronic effects (of substituents)11 Changes exerted by a substituent on a molecular property or molecular reactivity, often 12 distinguished as inductive (through-bond polarization), through-space electrostatics 13 (field effect), or resonance, but excluding steric.14 Note 1: The obsolete terms mesomeric and electromeric are discouraged.15 Note 2: Quantitative scales of substituent effects are available.16 See [126,173,191].17 See also polar effect.18 rev[3]19 revGB-revPOC2021 electrophile (n.), electrophilic (adj.)22 Reagent that forms a bond to its reaction partner (the nucleophile) by accepting both 23 bonding electrons from that partner.24 Note 1: Electrophilic reagents are Lewis acids.25 Note 2: "Electrophilic catalysis" is catalysis by Lewis acids.26 Note 3: The term "electrophilic" is also used to designate the apparent polar 27 character of certain radicals, as inferred from their higher relative reactivities with 28 reaction sites of higher electron density.29 See also electrophilicity.30 GB[3]3132 electrophilic substitution33 Heterolytic reaction in which the entering group adds to a nucleophile and in which the 34 leaving group, or electrofuge, relinquishes both electrons to its reaction partner, 35 whereupon it becomes another potential electrophile.36 Example (azo coupling):37 RN2

+ (electrophile) + H2NC6H5 (nucleophile) → p-H2NC6H4N=NR + H+ (electrofuge)38 Note: It is arbitrary to emphasize the electrophile and ignore the feature that this is 39 also a nucleophilic substitution, but the distinction depends on the electrophilic nature 40 of the reactant that is considered to react with the substrate.

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1 See also substitution.23 electrophilicity4 Relative reactivity of an electrophile toward a common nucleophile.5 Note: The concept is related to Lewis acidity. However, whereas Lewis acidity is 6 measured by relative equilibrium constants toward a common Lewis base, 7 electrophilicity is measured by relative rate constants for reactions of different 8 electrophilic reagents towards a common nucleophilic substrate.9 See [192,193,194].

10 See also nucleophilicity.11 rev[3]12 revGB-revPOC1314 element effect15 Ratio of the rate constants of two reactions that differ only in the identity of the element 16 in the leaving group.17 Example: kBr/kCl for the reaction of N3

– (azide) with CH3Br or CH3Cl.18 rev[3]19 revGB2021 elementary reaction22 Reaction for which no reaction intermediates have been detected or need to be 23 postulated in order to describe the chemical reaction on a molecular scale. An 24 elementary reaction is assumed to occur in a single step and to pass through only one 25 transition state or none.26 See [13].27 See also composite reaction, stepwise reaction.28 revGB[3]2930 elimination31 Reverse of an addition reaction.32 Note 1: In an elimination two groups (called eliminands) are lost, most often from two 33 different centres (1,2-elimination (-elimination) or 1,3-elimination, etc.) with 34 concomitant formation of an unsaturation (double bond, triple bond) in the molecule, or 35 formation of a ring.36 Note 2: If the groups are lost from a single carbon or nitrogen centre (1,1-elimination, 37 -elimination), the resulting product is a carbene or nitrene, respectively.38 revGB[3]3940 empirical formula

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1 List of the elements in a chemical species, with integer subscripts indicating the simplest 2 possible ratios of all elements.3 Note 1: In organic chemistry C and H are listed first, then the other elements in 4 alphabetical order.5 Note 2: This differs from the molecular formula, in which the subscripts indicate how 6 many of each element is included and which is an integer multiple of the empirical 7 formula. For example, the empirical formula of glucose is CH2O while its molecular 8 formula is C6H12O6.9 Note 3: The empirical formula is the information provided by combustion analysis,

10 which has been largely superseded by mass spectrometry, which provides the 11 molecular formula.121314 enantiomer15 One of a pair of stereoisomeric molecular entities that are non-superimposable mirror 16 images of each other.17 See [11].18 GB[3]1920 enantiomeric excess21 Absolute value of the difference between the mole fractions of two enantiomers:2223 x(e.e.) = |x+ – x –|2425 where x+ + x– = 1.26 Note: Enantiomeric excess can be evaluated experimentally from the observed 27 specific optical rotatory power []obs, relative to []max, the (maximum) specific optical 28 rotatory power of a pure enantiomer:2930 x(e.e.) 31 e.e. = | []obs/[]max |3233 and also by chiral chromatography, NMR, and MS methods.34 See [11].35 rev[3]36 GB3738 enantiomeric ratio39 mole fraction of one enantiomer in a mixture divided by the mole fraction of the other.40

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1 r(e.r.) = x+/x– or x–/x+

23 where x+ + x– = 14 See [11].5 rev[3]6 GB78 enantioselectivity9 See stereoselectivity.

10 GB[3]1112 encounter complex13 Complex of molecular entities produced at an encounter-controlled rate, and which 14 occurs as an intermediate in a reaction.15 Note 1: When the complex is formed from two molecular entities it is called an 16 "encounter pair". A distinction between encounter pairs and (larger) encounter 17 complexes may be relevant for mechanisms involving pre-association.18 Note 2: The separation between the entities is small compared to the diameter of a 19 solvent molecule.20 See also [9].21 rev[3]22 revGB-revPOC2324 encounter-controlled rate25 Rate of reaction corresponding to the rate at which the reacting molecular entities 26 encounter each other. This is also known as the "diffusion-controlled rate", since rates 27 of encounter are themselves controlled by diffusion rates (which in turn depend on the 28 viscosity of the medium and the dimensions of the reacting molecular entities).29 Note: At 25 °C in most solvents, including water, a bimolecular reaction that proceeds 30 at an encounter-controlled rate has a second-order rate constant of 109 to 1010 dm3 31 mol–1 s–1.32 See also microscopic diffusion control.33 GB[3]3435 ene reaction36 Addition of a compound with a double bond and an allylic hydrogen (the "ene") to a 37 compound with a multiple bond (the "enophile") with transfer of the allylic hydrogen and 38 a concomitant reorganization of the bonding.39 Example, with propene as the ene and ethene as the enophile.

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12 Note: The reverse is a "retro-ene" reaction.3 GB[3]45 energy of activation6 Ea or EA

7 Arrhenius energy of activation8 activation energy9 (SI unit: kJ mol–1)

10 Operationally defined quantity expressing the dependence of a rate constant on 11 temperature according to12

13 𝐸a(𝑇) = –𝑅 dln{

𝑘(𝑇)[𝑘] }

d(1𝑇)

1415 as derived from the Arrhenius equation, k(T) = A exp(EA/RT), where A is the pre-16 exponential factor and R the gas constant. As the argument of the ln function, k should 17 be divided by its units, i.e., by [k].18 Note 1: According to collision theory, the pre-exponential factor A is the frequency of 19 collisions with the correct orientation for reaction and Ea (or E0) is the threshold energy 20 that collisions must have for the reaction to occur.21 Note 2: The term Arrhenius activation energy is to be used only for the empirical 22 quantity as defined above. There are other empirical equations with different activation 23 energies, see [12].24 See [13].25 See also enthalpy of activation.26 rev[3]27 revGB2829 energy profile30 See Gibbs energy diagram, potential-energy profile.31 GB[3]3233 enforced concerted mechanism34 Situation where a putative intermediate possesses a lifetime shorter than a bond 35 vibration, so that the steps become concerted.36 See [195,196,197].37 rev[3]38 revGB

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12 enthalpy of activation (standard enthalpy of activation)3 ∆‡Ho

4 (SI unit: kJ mol–1)5 Standard enthalpy difference between the transition state and the ground state of the 6 reactants at the same temperature and pressure. It is related experimentally to the 7 temperature dependence of the rate coefficient k according to equation (1) for first-order 8 rate constants:9

10 ∆‡Hº = –R (1) { ∂ln (𝑘s ―1

𝑇 K ) ∂(1

𝑇) }𝑃

1112 and to equation (2) for second order rate coefficients13

14 ∆‡Hº = –R (2) {∂ln (𝑘 (mol dm3 s -1)𝑇 K )

∂(1𝑇) }

𝑃15

16 This quantity can be obtained, along with the entropy of activation ∆‡Sº, from the slope 17 and intercept of the linear least-squares fit of rate coefficients k to the equation18

19 ln = ∆‡Sº/R – ∆‡Hº/RT + ln[ (kB/J K-1)/(h/J s) ] (𝑘s -1

𝑇 K )20 for a first-order rate coefficient21 and

22 ln = ∆‡Sº/R – ∆‡Hº/R (1/T) + ln[ (kB/J K-1)/(h/J s) ] (𝑘 (mol dm3 s -1)𝑇 K )

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12 for a second-order rate coefficient where kB is Boltzmann constant, h is Planck constant, 3 and kB/h = 2.08366… 1010 K-1 s-1.4 Note 1: An advantage of the least-squares fit is that it can also give error estimates 5 for ∆‡Hº and ∆‡Sº.6 Note 2: It is also given by78 ∆‡Hº = RT2(∂ln k/[k]/∂T)P – RT = Ea – RT9

10 ∆‡Hº = RT2(∂ln{k}//∂T)P – RT1112 where Ea is the energy of activation, provided that the rate coefficients for reactions 13 other than first-order are expressed in temperature-independent concentration units 14 (e.g., mol kg–1, measured at a fixed temperature and pressure). The argument in a 15 logarithmic function should be of dimension 1. Thus k is divided by its units, i.e., by [k], 16 as in the alternative form, in terms of reduced variables.17 See also entropy of activation, Gibbs energy of activation.18 See [12,13].19 rev[3]20 revGB2122 entropy of activation, (standard entropy of activation)23 ∆‡Sº24 (SI unit: J mol–1 K–1)25 Standard entropy difference between the transition state and the ground state of the 26 reactants, at the same temperature and pressure. It is related to the Gibbs energy of 27 activation and enthalpy of activation by the equations2829 ∆‡Sº = (∆‡Hº –∆‡Gº)/T3031 provided that rate coefficients for reactions other than first-order reactions are 32 expressed in temperature-independent concentration units (e.g., mol dm–3, measured 33 at fixed temperature and pressure). The numerical value of S depends on the standard 34 state (and therefore on the concentration units selected).35 Note 1: It can also be obtained from the intercept of the linear least-squares fit of rate 36 coefficients k to the equation3738 ln(k/[k]/T[T]) = ∆‡Sº/R – ∆‡Hº/R (1/T) + ln(kB/h),3940 ln({k}/{T}) = ∆‡Sº/R – ∆‡Hº/R (1/T) + ln(kB/h),

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12 where kB/h = 2.08366… 1010 K-1 s-1. k should be divided by its units, i.e., by [k] = s-1 3 for first-order, and by [k] = (s-1 mol-1 dm3) for second-order rate coefficients and T should 4 be divided by its units [T] = K, as in the second equation, in terms of reduced variables. 5 Note 2: The information represented by the entropy of activation may alternatively 6 be conveyed by the pre-exponential factor A, which reflects the fraction of collisions 7 with the correct orientation for reaction (see energy of activation).8 See [12,13].9 rev[3]

10 revGB1112 epimer13 Diastereoisomer that has the opposite configuration at only one of two or more 14 tetrahedral stereogenic centres in the respective molecular entity.15 See [11]. 16 GB[3]1718 epimerization19 Interconversion of epimers by reversal of the configuration at one of the stereogenic 20 centres.21 See [11].22 GB[3]2324 equilibrium, chemical25 Situation in which reversible processes (processes that may be made to proceed in 26 either the forward or reverse direction by the infinitesimal change of one variable) have 27 reached a point where the rates in both directions are identical, so that the amount of 28 each species no longer changes.29 Note 1: In this situation the Gibbs energy, G, is a minimum. Also, the sum of the 30 chemical potentials of the reactants equals that of the products, so that

31 ∆rGº = –RT ln K32 where the thermodynamic equilibrium constant, K (of dimension 1), is the product of 33 product activities divided by the product of reactant activities.34 Note 2: In dilute solutions the numerical values of the thermodynamic activities may 35 be approximated by the respective concentrations.36 rev[3]3738 equilibrium control39 See thermodynamic control.40 GB[3]

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12 ET-value3 See Dimroth-Reichardt ET parameter, Z-value.4 GB[3]56 excess acidity7 See Bunnett-Olsen equations, Cox-Yates equation.8 GB[3]9

10 excimer (“excited dimer”)11 Complex formed by the interaction of an excited molecular entity with another identical 12 molecular entity in its ground state.13 Note: The complex is not stable in the ground state.14 See [9].15 See also exciplex.16 [3]1718 exciplex19 Electronically excited complex of definite stoichiometry that is non-bonding in the 20 ground state.21 Note: An exciplex is a complex formed by the noncovalent interaction of an excited 22 molecular entity with the ground state of a different molecular entity, but an excimer, 23 formed from two identical components, is often also considered to be an exciplex.24 See [9].25 See also excimer.26 GB[3]2728 excited state29 Condition of a system with energy higher than that of the ground state. This term is 30 most commonly used to characterize a molecular entity in one of its electronically 31 excited states, but may also refer to vibrational and/or rotational excitation in the 32 electronic ground state.33 See [9].34 GB[3]3536 EXSY (NMR exchange spectroscopy)37 Two-dimensional NMR technique producing cross peaks corresponding to site-to site 38 chemical exchange. The cross-peak amplitudes carry information about exchange 39 rates.40 See [198].

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12 extended Hammett equation3 Multiparameter extension of the Hammett equation for the description of substituent 4 effects.5 Note 1: The major extensions using two parameters (dual substituent-parameter 6 equations) were devised for the separation of inductive and steric effects (Taft equation) 7 or of inductive (or field) and resonance effects.8 Note 2: Other parameters may be added (polarizability, hydrophobicity…) when 9 additional substituent effects are operative.

10 See [191].11 See also Yukawa-Tsuno equation.12 [3]13 revGB-revPOC1415 external return (external ion-pair recombination)16 Recombination of free ions formed in an SN1 reaction (as distinguished from ion-pair 17 collapse).18 See ion-pair recombination.19 rev[3]20 revGB-revPOC2122 extrusion23 Transformation in which an atom or group Y connected to two other atoms or groups X 24 and Z is lost from a molecule, leading to a product in which X is bonded to Z, i.e.,25 X–Y–Z → X–Z + Y26 Example

2728 Note 1: When Y is a metal, this process is called a reductive elimination. 29 Note 2: The reverse of an extrusion is called an insertion.30 See also cheletropic reaction.31 GB[3]3233 field effect

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1 Experimentally observable substituent effect (on reaction rates, etc.) of intramolecular 2 electrostatic interaction between the centre of interest and a monopole or dipole, by 3 direct electric-field action through space rather than through bonds.4 Note 1: The magnitude of the field effect depends on the monopole charge or dipole 5 moment, on the orientation of the dipole, on the distance between the centre of interest 6 and the monopole or dipole, and on the effective dielectric constant that reflects how 7 the intervening bonds are polarized.8 Note 2: Although a theoretical distinction may be made between the field effect and 9 the inductive effect as models for the Coulomb interaction between a given site and a

10 remote monopole or dipole within the same entity, the experimental distinction between 11 the two effects has proved difficult, because the field effect and the inductive effect are 12 ordinarily influenced in the same direction by structural changes.13 Note 3: The substituent acts through the electric field that it generates, and it is an 14 oversimplification to reduce that interaction to that of a monopole or dipole.15 See also electronic effect, inductive effect, polar effect.16 See [172,191,199,200].17 rev[3]1819 flash photolysis20 Spectroscopic or kinetic technique in which an ultraviolet, visible, or infrared radiation 21 pulse is used to produce transient species.22 Note 1: Commonly, an intense pulse of short duration is used to produce a sufficient 23 concentration of transient species, suitable for spectroscopic observation. The most 24 common observation is of the absorption of the transient species (transient absorption 25 spectroscopy).26 Note 2: If only photophysical processes are involved, a more appropriate term would 27 be “pulsed photoactivation”. The term “flash photolysis” would be correct only if 28 chemical bonds are broken (the Greek “lysis” means dissolution or decomposition and 29 in general lysis is used to indicate breaking). However, historically, the name has been 30 used to describe the technique of pulsed excitation, independently of the process that 31 follows the excitation.32 See [9].3334 flash vacuum pyrolysis (FVP)35 Thermal reaction of a molecule by exposure to a short thermal shock at high 36 temperature, usually in the gas phase.37 Example:

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1 See [201,202,203,204].2 rev[3]3 revGB-revPOC45 fluxionality6 Property of a chemical species that undergoes rapid degenerate rearrangements 7 (generally detectable by methods that allow the observation of the behaviour of 8 individual nuclei in a rearranged chemical species, e.g., NMR, X-ray).9 Example: tricyclo[3.3.2.02,8]deca-3,6,9-triene (bullvalene), which has 1 209 600 (=

10 10!/3) interconvertible arrangements of the ten CH groups.11

1213 Note 1: Fluxionality differs from resonance, where no rearrangement of nuclear 14 positions occurs.15 Note 2: The term is also used to designate positional change among ligands of 16 complex compounds and organometallics. In these cases the change is not necessarily 17 degenerate.18 See also valence tautomerization.19 revGB[3]2021 force-field calculations22 See molecular mechanics calculation.23 GB2425 formal charge26 Quantity (omitted if zero) attached to each atom in a Lewis structure according to2728 Zformal = Nvalence – Nlonepairs – ½ Nbonds

2930 Examples: CH2=N+=N–, H3O+, (CH3)2C=N-O–

3132 Note: This formalism assumes that electrons in bonds are shared equally, regardless 33 of electronegativity.3435 Förster resonance-energy transfer (FRET)36 dipole-dipole excitation transfer

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1 Nonradiative mechanism for transfer of electronic excitation energy from one molecular 2 entity to another, distant one. It arises from the interaction between the transition dipole 3 moments of the two entities.4 See [9].56 fractionation factor, isotopic7 Ratio [x1(A)/x2(A)}/[x1(B)/x2(B)], where x is the mole fraction of the isotope designated 8 by the subscript, when the two isotopes are equilibrated between two different chemical 9 species A and B (or between specific sites A and B in the same chemical species).

10 Note 1: The term is most commonly met in connection with deuterium solvent isotope 11 effects, where the fractionation factor, symbolized by , expresses the ratio12 = [xD(solute)/xH(solute)}/[xD(solvent)/xH(solvent)]13 for the exchangeable hydrogen atoms in the chemical species (or sites) concerned.14 Note 2: The concept is also applicable to transition states.15 See [2].16 [3]1718 fragmentation19 (1) Heterolytic or homolytic cleavage of a molecule into more than two fragments, 20 according to the general reaction (where a, b, c, d, and X are atoms or groups of atoms)2122 a–b–c–d–X → (a–b)+ + c=d + X–

23 or a–b–c–d–X → (a–b)· + c=d + X24 Examples:25 Ph3C–COOH + H+ → Ph3C+ + C=O + H2O26 CH3N=NCH3 → CH3· + N2 + CH3

27 See [205].2829 (2) Breakdown of a radical or radical ion into a closed-shell molecule or ion and a 30 smaller radical31 Examples:32 (CH3)3C–O· → (CH3)2C=O + H3C·33 [ArBr]·– → Ar· + Br– (solution)34 [(CH3)3C–OH]·+ → (CH3)2C=OH+ + H3C· (gas-phase, following ionization)35 [3]36 revGB-revPOC3738 Franck-Condon Principle39 Approximation that an electronic transition is most likely to occur without change in 40 nuclear positions.

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1 Note 1: The resulting state is called a Franck–Condon state, and the transition 2 involved is called a vertical transition.3 Note 2: As a consequence, the intensity of a vibronic transition is proportional to the 4 square of the overlap integral between the vibrational wavefunctions of the two states 5 that are involved in the transition.6 See [9].7 GB89 free energy

10 The thermodynamic function Gibbs energy (symbol G) or Helmholtz energy (symbol A) 11 specifically defined as 1213 G = G(P,T) = H – TS14 and A = A(V,T) = U – TS1516 where H is enthalpy, U is internal energy, and S is entropy. The possibility of 17 spontaneous motion for a statistical distribution of an assembly of atoms (at absolute 18 temperature T above 0 K) is governed by free energy and not by potential energy.19 Note 1: The IUPAC recommendation is to use the specific terms Gibbs energy or 20 Helmholtz energy whenever possible. However, it is useful to retain the generic term 21 "free energy" for use in contexts where the distinction between (on the one hand) either 22 Gibbs energy or Helmholtz energy and (on the other hand) potential energy is more 23 important than the distinction between conditions either of constant pressure or of 24 constant volume; e.g., in computational modelling to distinguish between results of 25 simulations performed for ensembles under conditions of either constant P or constant 26 V at finite T and calculations based purely on potential energy.27 Note 2: Whereas motion of a single molecular entity is determined by the force acting 28 upon it, which is obtained as the negative gradient of the potential energy, motion for 29 an assembly of many molecular entities is determined by the mean force acting upon 30 the statistical distribution, which is obtained as the negative gradient of the potential of 31 mean force.32 revGB3334 free radical35 See radical.36 GB[3]3738 frontier orbitals39 Highest-energy Occupied Molecular Orbital (HOMO) and Lowest-energy Unoccupied 40 Molecular Orbital (LUMO) of a molecular entity.

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1 Note 1: These terms should be limited to doubly occupied orbitals, and not to a singly 2 occupied molecular orbital (sometimes designated as a SOMO), because HOMO and 3 LUMO are ambiguous for molecular orbitals that are half filled and thus only partly 4 occupied or unoccupied.5 Note 2: Examination of the mixing of frontier molecular orbitals of reacting molecular 6 entities affords an approach to the interpretation of reaction behaviour; this constitutes 7 a simplified perturbation theory of chemical behaviour.8 Note 3: In some cases a subjacent orbital (Next-to-Highest Occupied Molecular 9 Orbital (NHOMO) or a Second Lowest Unoccupied Molecular Orbital (SLUMO)) may

10 affect reactivity.11 See [8,206,207].12 See also SOMO, subjacent orbital.13 rev[3]14 revGB-revPOC 1516 frustrated Lewis acid-base pair17 Acid and base for which adduct formation is prevented by steric hindrance.

1819 See [208,209].2021 fullerene22 Molecular entity composed entirely of carbon, in the form of a hollow sphere, ellipsoid, 23 or tube.24 Note: Spherical fullerenes are also called buckyballs, and cylindrical ones are called 25 carbon nanotubes or buckytubes.26 See [40,210].2728 functional group29 Atom or group of atoms within molecular entities that are responsible for the 30 characteristic chemical reactions of those molecular entities. The same functional group 31 will undergo the same or similar chemical reaction(s) regardless of the size of the 32 molecule it is a part of. However, its relative reactivity can be modified by nearby 33 substituents.34 rev[3]35 revGB-revPOC

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12 gas-phase acidity3 Standard reaction Gibbs energy (∆rGº) change for the gas-phase reaction.

4 A–H → A– + H+

5 Note 1: The symbol often found in the literature is ∆acidG or GA.6 Note 2: The corresponding enthalpy ∆rHo is often symbolized by ∆acidH and called 7 “enthalpy of acidity” or deprotonation enthalpy, abbreviated DPE.8 See [211,212].9 rev[3]

10 revGB-revPOC1112 gas-phase basicity13 Negative of the standard reaction Gibbs energy (∆rGo) change for the gas-phase 14 reaction

15 B + H+ → BH+

16 Note: An acronym commonly used in the literature is GB. The corresponding 17 enthalpy ∆rHo is called proton affinity, PA, even though affinity properly refers to Gibbs 18 energy. Moreover, such acronyms are not accepted by IUPAC.19 See [213,214].20 revGB[3]2122 Gaussian orbital23 Function centred on an atom of the form (r) ∝ xiyjzkexp(–r2), used to approximate 24 atomic orbitals in the LCAO-MO method.25 See [8].2627 geminate pair28 Pair of molecular entities in close proximity within a solvent cage and resulting from 29 reaction (e.g., bond scission, electron transfer, group transfer) of a precursor.30 Note: Because of the proximity the pair constitutes only a single kinetic entity.31 See also ion pair, radical pair.32 rev[3]33 GB3435 geminate recombination36 Recombination reaction of a geminate pair.37 Example:

3839 rev[3]

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1 revGB-revPOC234 general acid catalysis5 Catalysis of a chemical reaction by Brønsted acids (which may include the solvated 6 hydrogen ion), where the rate of the catalysed part of the reaction is given by HAkHA[HA] 7 multiplied by some function of substrate concentrations.8 Note 1: General acid catalysis can be experimentally distinguished from specific 9 catalysis by hydrogen cations (hydrons) if the rate of reaction increases with buffer

10 concentration at constant pH and ionic strength.11 Note 2: The acid catalysts HA are unchanged by the overall reaction. This 12 requirement is sometimes relaxed, but the phenomenon is then properly called pseudo-13 catalysis.14 See also catalysis, catalytic coefficient, intramolecular catalysis, pseudo-catalysis, 15 specific catalysis.16 rev[3]17 revGB-revPOC1819 general base catalysis20 Catalysis of a chemical reaction by Brønsted bases (which may include the lyate ion), 21 where the rate of the catalysed part of the reaction is given by BkB[B] multiplied by 22 some function of substrate concentrations.23 See also general acid catalysis.24 GB[3]2526 Gibbs energy diagram 27 Diagram showing the relative standard Gibbs energies of reactants, transition states, 28 reaction intermediates, and products, in the same sequence as they occur in a chemical 29 reaction.30 Note 1: The abscissa expresses the sequence of reactants, products, reaction 31 intermediates, and transition states and is often an undefined "reaction coordinate" or 32 only vaguely defined as a measure of progress along a reaction path. In some 33 adaptations the abscissas are however explicitly defined as bond orders, Brønsted 34 exponents, etc.35 Note 2: These points are often connected by a smooth curve (a "Gibbs energy 36 profile", commonly referred to as a "free-energy profile", a terminology that is 37 discouraged), but experimental observation can provide information on relative 38 standard Gibbs energies only at the maxima and minima and not at the configurations 39 between them.

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1 Note 3: It should be noted that the use of standard Gibbs energies implies a common 2 standard state for all chemical species (usually 1 M for reactions in solution). Contrary 3 to statements in many textbooks, the highest point on a Gibbs energy diagram does not 4 automatically correspond to the transition state of the rate-limiting step. For example, 5 in a stepwise reaction consisting of two elementary reaction steps6 (1) A + B ⇄ C7 (2) C + D → E8 one of the two transition states must (in general) have a higher standard Gibbs energy 9 than the other. Under experimental conditions where all species have the standard-

10 state concentration, then the rate-limiting step is that whose transition state is of highest 11 standard Gibbs energy. However, under (more usual) experimental conditions where 12 all species do not have the standard-state concentration, then the value of the 13 concentration of D determines which reaction step is rate-limiting. However, if the 14 particular concentrations of interest, which may vary, are chosen as the standard state, 15 then the rate-limiting step is indeed the one of highest Gibbs energy.16

171819 See also potential energy profile, potential-energy surface, reaction coordinate.20 rev[3]21 revGB-revPOC2223 Gibbs energy of activation (standard free energy of activation)24 ∆‡Gº25 (SI unit: kJ mol–1)26 Standard Gibbs energy difference between the transition state of an elementary 27 reaction and the ground state of the reactants for that step. It is calculated from the rate 28 constant k via the absolute rate equation:29

30 ∆‡Gº = RT [ln( (kB/J K-1)/(h/J s) ) – ln(k/[k]/T/K)]31

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1 ∆‡Gº = RT [ln({kB}/{h}) – ln({k}/{T})]23 where kB is Boltzmann's constant, [k] are the units of k, and h Planck's constant. The 4 values of the rate constants, and hence the Gibbs energies of activation, depend upon 5 the choice of concentration units (or of the thermodynamic standard state).6 Note 1: For a complex stepwise reaction, composed of many elementary reactions, 7 ∆‡Gº, the observed Gibbs energy of activation (activation free energy), can be 8 calculated as RT [ln(kB/J K-1)/(h/J s) – ln(k’/[k’]/T/K)], where k’ is the observed rate 9 constant, k’ should be divided by its units, and T/K is the dimensionless absolute

10 temperature, of dimension 1, since the argument of a logarithmic function should be of 11 dimension 1, as expressed by the reduced variables in the second form of the equation.12 Note 2: Both ∆‡Gº and k’ are non-trivial functions of the rate constants and activation 13 energies of the elementary steps.14 See also enthalpy of activation, entropy of activation.15 rev[3]16 revGB-revPOC1718 graphene19 Allotrope of carbon, whose structure is a one-atom-thick planar sheet of sp2-bonded 20 carbon atoms in a honeycomb (hexagonal) crystal lattice.2122 ground state23 State of lowest Gibbs energy of a system [2].24 Note: In photochemistry and quantum chemistry the lowest-energy state of a 25 chemical entity (ground electronic state) is usually meant.26 See [9].27 See also excited state.28 rev[3]29 GB-revPOC3031 group32 See functional group, substituent.33 rev[3]34 revGB-revPOC3536 Grunwald-Winstein equation37 Linear Gibbs-energy relation (Linear free-energy relation)3839 lg(kS/k0) = mY40

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1 expressing the dependence of the rate of solvolysis of a substrate on the ionizing power 2 of the solvent, where the rate constant k0 applies to the reference solvent (80:20 3 ethanol-water by volume) and kS to the solvent S.4 Note 1: The parameter m is characteristic of the substrate and is assigned the value 5 unity for tert-butyl chloride (2-chloro-2-methylpropane). The value Y is intended to be a 6 quantitative measure of the ionizing power of the solvent S.7 Note 2: The equation was later extended [215] to the form89 lg(kS/k0) = mY + lN

1011 where N is the nucleophilicity of the solvent and l a susceptibility parameter 12 Note 3: The equation has also been applied to reactions other than solvolysis. 13 Note 4: For the definition of other Y-scales, see [216,217,218,219].14 See also Dimroth-Reichardt ET parameter, polarity.15 rev[3]16 revGB-revPOC1718 guest19 Organic or inorganic ion or molecule that occupies a cavity, cleft, or pocket within the 20 molecular structure of a host molecular entity and forms a complex with it or that is 21 trapped in a cavity within the crystal structure of a host, but with no covalent bond being 22 formed.23 See also crown ether, cryptand, inclusion compound.24 GB[3]2526 half-life27 t1/2

28 (SI unit: s)29 Time required for the concentration of a particular reacting chemical species to fall to 30 one-half of its initial value.31 Note 1: Half-life is independent of initial concentration only for a first-order process.32 Note 2: For first-order reactions t1/2 = ln2, where is the lifetime.33 See also lifetime.34 rev[3]35 revGB-revPOC3637 halochromism38 Colour change that occurs on addition of acid or base to a solution of a compound as a 39 result of chemical reaction, or on addition of a salt as a result of changing the solvent 40 polarity.

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1 See [220,221].2 GB[3]34 halogen bond5 Association between a Lewis-acidic halogen atom in a molecular entity and a Lewis-6 basic region in another, or the same, molecular entity, which acts as an electron-pair 7 donor, such that the balance of forces of attraction and repulsion results in net 8 stabilization.9 Note 1: Typical halogen-bond donors are I2, Br2, ICN, and ICΞCH.

10 Note 2: This is analogous to a hydrogen bond, in which the H is the acidic atom.11 Note 3: The interaction provides a stabilization of a few kJ mol–1.12 See [222,223,224,225].131415 Hammett equation (Hammett relation)16 Equation of the form1718 lg {kX} = X + lg {k0}19 or lg {KX} = X + lg {K0}2021 expressing the influence of meta and para substituents X on the reactivity of the 22 functional group Y in the benzene derivatives m- and p-XC6H4Y, where {kX} and {KX} 23 are the numerical values of the rate or equilibrium constant, respectively, for the 24 reactions of m- and p-XC6H4Y, X is the substituent constant characteristic of m- or p-25 X, and is the reaction constant characteristic of the given reaction of Y. The values lg 26 {k0} and lg {K0} are intercepts for graphical plots of lg {kX} or lg {KX}, respectively, against 27 X for all substituents X (including X = H, if present). 28 Note 1: It is very common in the literature to find either of these equations written 29 without the curly brackets denoting a reduced rate or equilibrium constant, i.e., k or K 30 divided by its unit, [k] or [K], respectively. 31 Note 2: The Hammett equation is most frequently used to obtain the value of which, 32 as the slope of a graph or least-squares fit, is independent of the choice of units for kX 33 or KX. If the equation is used to estimate the unknown value of a rate or equilibrium 34 constant for an X-substituted compound based upon the known value of X, the ordinate 35 must be multiplied by [kX] or [KX] in order to obtain the value with its appropriate units.36 Note 3: The equation is often encountered in a form with kH or KH incorporated into 37 the logarithm on the left-hand side, where kH or KH corresponds to the reaction of parent 38 C6H5Y, with X = H;3940 lg(kX/kH) = X

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1 or lg(KX/KH) = X

23 This form satisfies the requirement that arguments of the lg function must be of 4 dimension 1, but it would suggest a one-parameter linear least-squares fit, whereas lg 5 k0, lg K0, lg k0/[k0] and lg K0/[K0], in the other forms correctly represent the intercept in 6 a two-parameter linear least-squares fit of lg k, lg K, lg k/[k], or lg K/[K] vs. X. 7 See [21,22,126,191,226].8 See also extended Hammett equation, Taft equation, Yukawa-Tsuno equation, -9 constant, -value.

10 rev[3]11 revGB1213141516 Hammond postulate (Hammond-Leffler principle)17 Hypothesis that, when a transition state leading to a high-energy reaction intermediate 18 (or product) has nearly the same energy as that intermediate (or product), the two are 19 interconverted with only a small reorganization of molecular structure.20 Note 1: Essentially the same idea is sometimes referred to as "Leffler's assumption", 21 namely, that the transition state bears a greater resemblance to the less stable species 22 (reactant or reaction intermediate/product). Many textbooks and physical organic 23 chemists, however, express the idea in Leffler's form (couched in terms of Gibbs 24 energies) but attribute it to Hammond (whose original conjecture concerns structure). 25 Note 2: As a corollary, it follows that a factor stabilizing a reaction intermediate will 26 also stabilize the transition state leading to that intermediate.27 Note 3: If a factor stabilizes a reaction intermediate (or reactant or product), then the 28 position of the transition state along the minimum-energy reaction path (MERP) for that 29 elementary step moves away from that intermediate, as shown in the energy diagram 30 (a) below left, where the transition state moves from ‡ to ‡’ when the species to the 31 right is stabilized. This behaviour is often called a Hammond effect and is simply a 32 consequence of adding a linear perturbation to the parabola.33

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12 Note 4: If a structure lying off the MERP is stabilized, then the position of the 3 transition state moves toward that structure, as shown in the energy diagram (b) above 4 right, where the transition state moves from ‡ to ‡’. This behaviour is often called anti-5 Hammond and arises because the transition state is a maximum along the MERP but 6 a minimum perpendicular to it.7 See [227,228,229,230].8 See Leffler's relation, More O'Ferrall – Jencks diagram, parallel effect, perpendicular 9 effect.

10 rev[3]11 revGB-revPOC1213 Hansch constant14 Measure of the contribution of a substituent to the partition ratio of a solute, defined as1516 X = lg(PX/PH)1718 where PX is the partition ratio for the compound with substituent X and PH is the partition 19 constant for the parent.20 See [231,232].21 rev[3]22 revGB-revPOC2324 hapticity25 Topological description for the number of contiguous atoms of a ligand that are bonded 26 to a central metal atom.27 Note: The hapticity is indicated as a superscript following the Greek letter .28 Example: (C5H5)2Fe (ferrocene) = bis(5-cyclopentadienyl)iron, where 5 can be read 29 as eta-five or pentahapto.30 See [29].31

(a) (b)

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1 hard acid, base2 Lewis acid with an acceptor centre or Lewis base with a donor centre (e.g., an oxygen 3 atom) of low polarizability.4 Note 1: A high polarizability characterizes a soft acid or base.5 Note 2: Whereas the definition above is qualitative, a theoretically consistent 6 definition of hardness, in eV, is as half the second derivative of the calculated energy 7 E with respect to N, the number of electrons, at constant potential due to the nuclei.8

9 𝜂 = 12 (∂2𝐸

∂𝑁2)𝜈

1011 Note 3: Other things being equal, complexes of hard acids with hard bases or of soft 12 acids with soft bases have an added stabilization (sometimes called the HSAB rule). 13 For limitations of the HSAB rule, see [36,233].14 See [234,235,236].15 rev[3]16 revGB-revPOC1718 HBA solvent19 Strongly or weakly basic solvent capable of acting as a hydrogen-bond acceptor (HBA) 20 and forming strong intermolecular solute-solvent hydrogen bonds.21 Note: A solvent that is not capable of acting as a hydrogen-bond acceptor is called 22 a non-HBA solvent.23 See [17].2425 HBD solvent (Hydrogen-Bond Donating solvent, also dipolar HBD solvent and protic 26 solvent)27 Solvent with a sizable permanent dipole moment that bears suitably acidic hydrogen 28 atoms to form strong intermolecular solvent-solute hydrogen bonds.29 Note: A solvent that is not capable of acting as hydrogen-bond donor is called a non-30 HBD solvent (formerly aprotic solvent).31 See [17,155].3233 heat capacity of activation34 CP

‡

35 (SI unit: J mol–1 K–1)36 Temperature coefficient of ∆‡H (enthalpy of activation) or ∆‡S (entropy of activation) at 37 constant pressure according to the equations:3839 CP

‡ = (∂∆‡H/∂T)P = T(∂∆‡S/∂T)P

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12 See [237]. 3 rev[3]4 revGB-revPOC56 Henderson-Hasselbalch equation7 Equation of the form89 pH = pKa – lg([HA]/[A–])

1011 relating the pH of a buffer solution to the ratio [HA]/[A–] and the dissociation constant of 12 the acid Ka.13 See [238].14 rev[3]15 revGB-revPOC1617 heterobimetallic complex18 Metal complex having two different metal atoms or ions.19 rev[3]20 revGB2122 heteroleptic23 Characteristic of a transition metal or Main Group compound having more than one type 24 of ligand.25 See also homoleptic.26 GB[3]272829 heterolysis, heterolytic bond fission30 Cleavage of a covalent bond so that both bonding electrons remain with one of the two 31 fragments between which the bond is broken, e.g.,

3233 See also heterolytic bond-dissociation energy, homolysis.34 GB[3]3536 heterolytic bond-dissociation energy37 Energy required to break a given bond of a specific compound by heterolysis.38 Note: For the dissociation of a neutral molecule AB in the gas phase into A+ and B– 39 the heterolytic bond-dissociation energy D(A+B–) is the sum of the homolytic bond-

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1 dissociation energy, D(A–B), and the adiabatic ionization energy of the radical A· minus 2 the electron affinity of the radical B·.3 GB[3]45 high-throughput screening6 Automated method to quickly assay large libraries of chemical species for the affinity of 7 small organic molecules toward a target of interest.8 Note: Currently more than 105 different compounds can be tested per day.9 See [239].

1011 highest occupied molecular orbital (HOMO)12 Doubly filled molecular orbital of highest energy.13 Note: Examination of the HOMO can predict whether an electrocyclic reaction is 14 conrotatory or disrotatory.15 See also frontier orbitals.1617 Hildebrand solubility parameter [symbol , derived unit: Pa1/2 = (kg m–1 s–2)1/2]18 Ability of a solvent to dissolve a non-electrolyte, defined as the square root of the 19 solvent's cohesive energy density (also called cohesive pressure, equal to the energy 20 of vaporization divided by the solvent’s molar volume and corresponding to the energy 21 necessary to create a cavity in the solvent).22 See [240].23 rev[3]24 revGB-revPOC2526 Hofmann rule27 Observation that when two or more alkenes can be produced in a -elimination reaction, 28 the alkene having the smallest number of alkyl groups attached to the double bond 29 carbon atoms is the predominant product.30 Note: This orientation is observed in elimination reactions of quaternary ammonium 31 salts and tertiary sulfonium (sulfanium) salts, and in certain other cases where there is 32 steric hindrance.33 See [241].34 See also Saytzeff rule.35 rev[3]36 revGB-revPOC3738 HOMO39 (1) Acronym for Highest Occupied Molecular Orbital.40 See also frontier orbitals.

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1 (2) Prefix (in lower case) used to indicate a higher homologue of a compound, as 2 homocysteine for HSCH2CH2CH(NH2)COOH, the homologue of cysteine, 3 HSCH2CH(NH2)COOH.4 rev[3]5 GB67 homoaromatic8 Showing features of aromaticity despite a formal discontinuity in the overlap of a cyclic 9 array of p orbitals resulting from the presence of an sp3-hybridized atom at one or

10 several positions within the cycle, in contrast to an aromatic molecule, where there is a 11 continuous overlap of p orbitals over a cyclic array.12 Note 1: Homoaromaticity arises because p-orbital overlap can bridge an sp3 centre.13 Note 2: Pronounced homoaromaticity is not normally associated with neutral 14 molecules, but mainly with ionic species, e.g., the "homotropylium" cation, C8H9

+,

1516 Note 3: In bis-, tris-, (etc.) homoaromatic species, two, three, (etc.) single sp3 centres 17 separately interrupt the π-electron system.18 GB[3]1920 homodesmotic reaction21 Subclass of isodesmic reactions in which reactants and products contain not only the 22 same number of carbon atoms in each state of hybridization but also the same number 23 of each CHn group joined to n hydrogen atoms.24 Examples:2526 cyclo-(CH2)3 + 3 CH3CH3 ⇄ 3 CH3CH2CH3 [3CH2, 6CH3]27 C6H6 (benzene) + 3 CH2=CH2 ⇄ 3 CH2=CH–CH=CH2 [6CH, 6CH2]2829 Note: The definition may be extended to molecules with heteroatoms.30 See [8].31 rev[3]32 GB3334 homoleptic35 Characteristic of a transition-metal or Main Group compound having only one type of 36 ligand, e.g., Ta(CH3)5

37 See also heteroleptic.

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1 GB[3]23 homolysis4 Cleavage of a bond so that each of the molecular fragments between which the bond 5 is broken retains one of the bonding electrons.6 Note 1: A unimolecular reaction involving homolysis of a bond not forming part of a 7 cyclic structure in a molecular entity containing an even number of (paired) electrons 8 results in the formation of two radicals:

910 Note 2: Homolysis is the reverse of colligation.11 See also bond dissociation energy, heterolysis.12 GB[3]1314 host15 Molecular entity that forms complexes (adducts) with organic or inorganic guests, or a 16 chemical species that can accommodate guests within cavities of its crystal structure.17 Examples include cryptands and crown ethers (where there are ion/dipole attractions 18 between heteroatoms and cations), hydrogen-bonded molecules that form clathrates 19 (e.g., hydroquinone or water), and host molecules of inclusion compounds (e.g., urea 20 or thiourea), where intermolecular forces and hydrophobic interactions bind the guest 21 to the host molecule.22 rev[3]23 revGB-revPOC2425 Hückel molecular orbital (HMO) theory26 Simplest molecular orbital theory of -conjugated molecular systems. It uses the 27 following approximations: -electron approximation; LCAO representation of the 28 molecular orbitals; neglect of electron-electron and nuclear-nuclear repulsions. The 29 diagonal elements of the effective Hamiltonian (Coulomb integrals) and the off-diagonal 30 elements for directly bonded atoms (resonance integrals) are taken as empirical 31 parameters, all overlap integrals being neglected.32 See [8].33 rev[3]34 GB3536 Hückel (4n + 2) rule37 Principle that monocyclic planar (or almost planar) systems of trigonally (or sometimes 38 digonally) hybridized atoms that contain (4n + 2) electrons (where n is an integer, 39 generally 0 to 5) exhibit aromatic character.

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1 Note 1: This rule is derived from Hückel MO theory calculations on planar monocyclic 2 conjugated hydrocarbons (CH)m where integer m is at least 3, according to which (4n + 3 2) electrons are contained in a closed-shell system.4 Examples:

56 Note 2: Planar systems containing 4n electrons (such as cyclobutadiene and 7 cyclopentadienyl cation) are antiaromatic.8 Note 3: Cyclooctatetraene, with 8 electrons, is nonplanar and therefore neither 9 aromatic nor antiaromatic but nonaromatic.

10 See [8].11 See also conjugation, Möbius aromaticity.12 rev[3]13 revGB-revPOC1415 hybrid orbital16 Atomic orbital constructed as a linear combination of atomic orbitals on an atom.17 Note 1: Hybrid orbitals are often used to describe the bonding in molecules 18 containing tetrahedral (sp3), trigonal (sp2), and digonal (sp) atoms, whose bonds are 19 constructed using 1:3, 1:2, or 1:1 combinations, respectively, of s and p atomic orbitals.20 Note 2: Construction of hybrid orbitals can also include d orbitals, as on an octahedral 21 atom, with d2sp3 hybridization.22 Note 3: Integer ratios are not necessary, and the general hybrid orbital made from s 23 and p orbitals can be designated as sp.24 GB2526 hydration27 Addition of water or of the elements of water (i.e., H and OH) to a molecular entity or to 28 a chemical species.29 Example: hydration of ethene:30 CH2=CH2 + H2O CH3CH2OH31 Note: In contrast to aquation, hydration, as in the incorporation of waters of 32 crystallization into a protein or in the formation of a layer of water on a nonpolar surface, 33 does not necessarily require bond formation.34 See [48].35 See also aquation.36 rev[3]

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1 revGB-revPOC234 hydrogen bond5 An attractive interaction between a hydrogen atom from a molecule or a molecular 6 fragment X–H in which X is more electronegative than H, and an atom or a group of 7 atoms in the same or a different molecule, in which there is evidence of bond formation.8 Note 1: Both electronegative atoms are usually (but not necessarily) from the first 9 row of the Periodic Table, i.e., N, O, or F.

10 Note 2: A hydrogen bond is largely an electrostatic interaction, heightened by the 11 small size of hydrogen, which permits proximity of the interacting dipoles or charges.12 Note 3: Hydrogen bonds may be intermolecular or intramolecular.13 Note 4: With few exceptions, usually involving hydrogen fluoride and fluoride and 14 other ions, the associated energies are less than 20-25 kJ mol-1.15 Note 5: Hydrogen bonds are important for many chemical structures, giving rise to 16 the attraction between H2O molecules in water and ice, between the strands of DNA, 17 and between aminoacid residues in proteins.18 See [242,243].19 rev[3]20 revGB-revPOC2122 hydrolysis23 Solvolysis by water, generally involving the rupture of one or more bonds in the reactant 24 and involvement of water as nucleophile or base.

25 Example: CH3C(=O)OCH2CH3 + H2O ➛ CH3C(=O)OH + CH3CH2OH26 rev[3]27 revGB-revPOC2829 hydron30 General name for the ion H+ either in natural abundance or where it is not desired to 31 distinguish between the isotopes, as opposed to proton for 1H+, deuteron for 2H+ and 32 triton for 3H+.33 See [244].34 GB[3]3536 hydronation37 Attachment of the ion H+ either in natural abundance or where it is not desired to 38 distinguish between the isotopes.3940 hydrophilicity

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1 Capacity of a molecular entity or of a substituent to undergo stabilizing interactions with 2 polar solvents, in particular with water and aqueous mixtures, to an extent greater than 3 with a nonpolar solvent.4 rev[3]5 revGB67 hydrophobic interaction8 Tendency of lipophilic hydrocarbon-like groups in solutes to form intermolecular 9 aggregates in an aqueous medium.

10 Note: The name arises from the attribution of the phenomenon to the apparent 11 repulsion between water and hydrocarbons. However, the phenomenon is more 12 properly attributed to the effect of the hydrocarbon-like groups to avoid disrupting the 13 favorable water-water interactions.14 GB[3]1516 hyperconjugation17 Delocalization of electrons between bonds and a network.18 Note 1: The concept of hyperconjugation is often applied to carbenium ions and 19 radicals, where the interaction is between bonds and an unfilled or partially filled p or 20 orbital. Resonance illustrating this for the tert-butyl cation is:

2122 Note 2: A distinction is made between positive hyperconjugation, as above, and 23 negative hyperconjugation, where the interaction is between a filled or orbital and 24 adjacent antibonding * orbitals, as for example in the fluoroethyl anion (2-fluoroethan-25 1-ide).

2627 Note 3: Historically, conjugation involves only bonds, and hyperconjugation is 28 considered unusual in involving bonds.29 See [245,246,247,248].30 See also , , delocalization.31 rev[3]32 revGB-revPOC

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12 hypercoordinated3 Feature of a main-group atom with a coordination number greater than four.4 Example: the pentacoordinate carbon in the carbonium ion CH5

+, where three C–H 5 bonds may be regarded as two-electron bonds and the two electrons in the remaining 6 CH2 fragment are delocalized over three atoms. Likewise, both hydrogens in the CH2 7 fragment are hypercoordinated.8 See [2,8].9 See also agostic.

10 rev[3]11 rev GB-revPOC1213 hypervalency14 Ability of an atom in a molecular entity to expand its valence shell beyond the limits of 15 the Lewis octet rule.16 Examples: PF5, SO3, iodine(III) compounds, and pentacoordinate carbocations 17 (carbonium ions).18 Note: Hypervalent compounds are more common for the second- and subsequent-19 row elements in groups 15-18 of the periodic table. A proper description of the 20 hypervalent bonding implies a transfer of electrons from the central (hypervalent) atom 21 to the nonbonding molecular orbitals of the attached ligands, which are usually more 22 electronegative.23 See [8].24 See also valence.25 rev[3]26 GB2728 hypsochromic shift29 Shift of a spectral band to higher frequency (shorter wavelength) upon substitution or 30 change in medium.31 Note: This is informally referred to as a blue shift and is opposite to a bathochromic 32 shift ("red shift"), but these historical terms are discouraged because they apply only to 33 visible transitions.34 See [9].35 rev[3]36 GB3738 identity reaction39 Chemical reaction whose products are chemically identical with the reactants40 Examples:

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1 (i) bimolecular exchange reaction of CH3I with I–2 (ii) proton transfer between NH4

+ and NH3

3 (iii) electron transfer between manganate(VI) MnO42– and permanganate MnO4

–.4 See also degenerate rearrangement.5 rev[3]6 revGB78 imbalance9 Feature that reaction parameters characterizing different bond-forming or bond-

10 breaking processes in the same reaction change to different extents as the transition 11 state is approached (along some arbitrarily defined reaction path).12 Note: Imbalance is common in reactions such as elimination, addition, and other 13 complex reactions that involve proton (hydron) transfer.14 Example: the nitroalkane anomaly, where the Brønsted exponent for hydron 15 removal is smaller than the Brønsted for the nitroalkane as acid, because of 16 imbalance between the extent of bond breaking and the extent of resonance 17 delocalization in the transition state.18 See [249].19 See also Brønsted relation, synchronization (principle of nonperfect 20 synchronization), synchronous.21 rev[3]22 GB2324 inclusion compound (inclusion complex)25 Complex in which one component (the host) forms a cavity or, in the case of a crystal, 26 a crystal lattice containing spaces in the shape of long tunnels or channels in which 27 molecular entities of a second chemical species (the guest) are located.28 Note: There is no covalent bonding between guest and host, the attraction being 29 generally due to van der Waals forces. If the spaces in the host lattice are enclosed on 30 all sides so that the guest species is "trapped" as in a cage, such compounds are known 31 as clathrates or cage compounds".32 See [40].33 GB[3]3435 induction period36 Initial slow phase of a chemical reaction whose rate later accelerates.37 Note: Induction periods are often observed with radical reactions, but they may also 38 occur in other reactions, such as those where a steady-state concentration of the 39 reactants is not established immediately.40 See [13].

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1 GB[3]23 inductive effect4 Experimentally observable effect (on rates of reaction, etc.) of a substituent through 5 transmission of charge through a chain of atoms by electrostatics.6 Note: Although a theoretical distinction may be made between the field effect and 7 the inductive effect as models for the Coulomb interaction between a given site within 8 a molecular entity and a remote monopole or dipole within the same entity, the 9 experimental distinction between the two effects has proved difficult (except for

10 molecules of peculiar geometry, which may exhibit "reversed field effects"), because 11 the inductive effect and the field effect are ordinarily influenced in the same direction by 12 structural changes.13 See [171,172,250].14 See also field effect, polar effect.15 rev[3]16 revGB-revPOC1718 inert19 Stable and unreactive under specified conditions.20 GB[3]2122 inhibition23 Decrease in rate of reaction brought about by the addition of a substance (inhibitor), by 24 virtue of its effect on the concentration of a reactant, catalyst, or reaction intermediate.25 For example, molecular oxygen or 1,4-benzoquinone can act as an inhibitor in many 26 chain reactions involving radicals as intermediates by virtue of its ability to act as a 27 scavenger toward those radicals.28 Note: If the rate of a reaction in the absence of inhibitor is vo and that in the presence 29 of a certain amount of inhibitor is v, the degree of inhibition (i) is given by3031 i = (vo – v)/vo

3233 See also mechanism-based inhibition.34 GB[3]3536 initiation37 Reaction or process generating radicals (or some other reactive reaction intermediates) 38 which then induce a chain reaction or catalytic cycle.39 Example: In the chlorination of alkanes by a radical mechanism the initiation step 40 may be the dissociation of molecular chlorine.

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1 rev[3]2 revGB-revPOC34 inner-sphere (electron transfer)5 Feature of an electron transfer between two metal centres that in the transition state 6 share a ligand or atom in their coordination shells.7 Note: The definition has been extended to any situation in which the interaction 8 between the electron-donor and electron-acceptor centres in the transition state is 9 significant (>20 kJ mol–1).

10 See [9].11 See also outer-sphere electron transfer.12 GB[3]1314 insertion15 Chemical reaction or transformation of the general type1617 X–Z + Y X–Y–Z1819 in which the connecting atom or group Y replaces the bond joining the parts X and Z of 20 the reactant XZ.21 Example: carbene insertion reaction2223 R3C–H + H2C: R3C–CH3

2425 Note: The reverse of an insertion is called an extrusion.26 GB[3]2728 intermediate (reactive intermediate)29 Molecular entity in a stepwise chemical reaction with a lifetime appreciably longer than 30 a molecular vibration (corresponding to a local potential energy minimum of depth 31 greater than RT, and thus distinguished from a transition state) that is formed (directly 32 or indirectly) from the reactants and reacts further to give (directly or indirectly) the 33 products of a chemical reaction.34 See also elementary reaction, reaction step, stepwise reaction. 35 GB[3]3637 intermolecular38 (1) Descriptive of any process that involves a transfer (of atoms, groups, electrons, etc.) 39 or interactions between two or more molecular entities.40 (2) Relating to a comparison between different molecular entities.

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1 See also intramolecular.2 GB[3]34 internal return5 See ion-pair recombination.6 rev[3]7 revGB89 intramolecular

10 (1) Descriptive of any process that involves a transfer (of atoms, groups, electrons, etc.) 11 or interactions between different parts of the same molecular entity.12 (2) Relating to a comparison between different groups within the same molecular entity.13 See also intermolecular.14 GB[3]1516 intramolecular catalysis17 Acceleration of a chemical transformation at one site of a molecular entity through the 18 involvement of another functional ("catalytic") group in the same molecular entity, 19 without that group appearing to have undergone change in the reaction product.20 Note 1: The use of the term should be restricted to cases for which analogous 21 intermolecular catalysis by a chemical species bearing that catalytic group is 22 observable.23 Note 2: Intramolecular catalysis can be detected and expressed in quantitative form 24 by a comparison of the reaction rate with that of a comparable model compound in 25 which the catalytic group is absent, or by measurement of the effective molarity of the 26 catalytic group.27 See also effective molarity, neighbouring group participation.28 GB[3]2930 intrinsic barrier, Δ‡G0

31 Gibbs energy of activation in the limiting case where ΔrGo = 0, i.e., when the effect of 32 thermodynamic driving force is eliminated, as in an identity reaction, X* + AX → X*A + 33 X, where A may be an atom, ion, or group of atoms or ions, and where the equilibrium 34 constant K is equal to 1.35 Note: According to the Marcus equation, originally developed for outer-sphere 36 electron transfer reactions, the intrinsic barrier is related to λ, the reorganization energy 37 of the reaction, by the equation3839 Δ‡Gº = /440

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1 For a non-identity reaction, Y + AX → YA + X, the intrinsic barrier Δ‡Gº(Y,X) is estimated 2 as ½[Δ‡G(X,X) + Δ‡G(Y,Y)], where the latter two terms are the Gibbs energies of 3 activation of the identity reactions X* + AX → X*A + X and Y* + AY → Y*A + Y,4 respectively.5 See [251,252,253,254].6 rev[3]7 revGB-revPOC89 intrinsic reaction coordinate (IRC)

10 Minimum-energy path leading from the saddle point (corresponding to the transition 11 structure) on the potential-energy surface for an elementary reaction, obtained by 12 tracing the steepest descent in mass-weighted coordinates in both directions.13 Note: The IRC is mathematically well defined, in contrast to the (generally) vague 14 reaction coordinate. Strictly, the IRC is a specific case of a minimum-energy reaction 15 path, and its numerical value at any point along this path is usually taken to be zero at 16 the saddle point, positive in the direction of the products, and negative in the direction 17 of the reactants.18 See [8,255].19 rev[3]20 revGB2122 inverted micelle (reverse micelle) 23 Association colloid formed reversibly from surfactants in non-polar solvents, leading to 24 aggregates in which the polar groups of the surfactants are concentrated in the interior 25 and the lipophilic groups extend toward and into the non-polar solvent.26 Note: Such association is often of the type27 Monomer ⇄ Dimer ⇄ Trimer ⇄ ··· ⇄ n-mer28 and a critical micelle concentration is consequently not observed.29 GB[3]3031 ion pair32 Pair of oppositely charged ions held together by coulomb attraction without formation 33 of a covalent bond.34 Note 1: Experimentally, an ion pair behaves as one unit in determining conductivity, 35 kinetic behaviour, osmotic properties, etc.36 Note 2: Following Bjerrum, oppositely charged ions with their centres closer together 37 than a distance38

39 d = 𝑍 + 𝑍 ― 𝑒2

4π𝜀0𝜀r𝑘B𝑇

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12 are considered to constitute an ion pair. Here Z+ and Z– are the charge numbers of the 3 ions, e is the elementary charge, ε0 is the vacuum permittivity, εr is the relative 4 permittivity ("dielectric constant") of the medium, kB is Boltzmann's constant, and T is 5 the absolute temperature. This is the distance at which the Coulomb energy equals the 6 thermal energy, and e2/(40kB) is approximately equal to 1.671 105 m K-1.7 Note 3: An ion pair, the constituent ions of which are in direct contact (and not 8 separated by intervening solvent or by other neutral molecule) is designated as a "tight 9 ion pair" (or "intimate" or "contact ion pair"). A tight ion pair of R+ and X– is symbolically

10 represented as R+X–.11 Note 4: By contrast, an ion pair whose constituent ions are separated by one or 12 several solvent or other neutral molecules is described as a "loose ion pair", 13 symbolically represented as R+||X–. The components of a loose ion pair can readily 14 interchange with other free or loosely paired ions in the solution. This interchange may 15 be detectable (e.g., by isotopic labelling) and thus afford an experimental distinction 16 between tight and loose ion pairs.17 Note 5: A further conceptual distinction has sometimes been made between two 18 types of loose ion pairs. In "solvent-shared ion pairs" for which the ionic constituents of 19 the pair are separated by only a single solvent molecule, whereas in "solvent-separated 20 ion pairs" more than one solvent molecule intervenes. However, the term "solvent-21 separated ion pair" must be used and interpreted with care since it has also widely been 22 used as a less specific term for "loose" ion pair.23 See [256].24 See also common-ion effect, dissociation, ion-pair return, special salt effect.25 GB[3]2627 ion-pair recombination (formerly ion-pair return)28 Recombination of a pair of ions R+ and X– formed from ionization of RX.29 Note: Ion-pair recombination can be distinguished as external or internal, depending 30 on whether the ion pair did or did not undergo dissociation to free ions.31 See ion pair.32 rev[3]33 revGB-revPOC34353637 ionic liquid (ionic solvent, molten salt)38 Liquid that consists exclusively or almost exclusively of equivalent amounts of 39 oppositely charged ions.40 Note 1: In practice the ions are monocations and monoanions.

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1 Note 2: Ionic liquids that are liquid at or around room temperature are called room-2 temperature ionic liquids (RTILs).3 Note 3: The term ionic liquid has been often restricted to those water-free liquids that 4 have melting points (or glass-transition temperatures) below 100 °C, following a 5 definition given by Walden [257], who prepared the first ionic liquid, ethylammonium

6 nitrate, CH3CH2NH3+ NO3

−, mp. 13-14 °C, for conductivity measurements.7 Note 4: The terminology for ionic liquids is not yet settled, as stated by Welton [258]. 8 Room-temperature ionic liquid, non-aqueous ionic liquid, molten salt, liquid organic salt, 9 and fused salt are often synonymous.

10 See [259,260,261].1112 ionic strength13 I14 (In concentration basis, Ic, SI unit: mol m–3, more commonly mol dm–3 or mol L–1, in 15 molality basis, Im , SI unit: mol kg-1)16 In concentration basis: Ic = 0.5 iciZi

2, in which ci is the concentration of a fully 17 dissociated electrolyte in solution and Zi the charge number of ionic species i.18 In molality basis: Im = 0.5 imiZi

2, in which mi is the molality of a fully dissociated 19 electrolyte in solution and Zi the charge number of ionic species i.20 rev[3]21 revPOC2223 ionization24 Generation of one or more ions.25 Note 1: This may occur, e.g., by loss or gain of an electron from a neutral molecular 26 entity, by the unimolecular heterolysis of that entity into two or more ions, or by a 27 heterolytic substitution reaction involving neutral molecules, such as28 M → M+• + e– (photoionization, electron ionization in mass spectrometry)29 SF6 + e– → SF6

–• (electron capture)30 Ph3CCl → Ph3C+ Cl– (ion pair, which may dissociate to free ions)31 CH3COOH + H2O → H3O+ + CH3CO2

– (dissociation)32 Note 2: In mass spectrometry ions may be generated by several methods, including 33 electron ionization, photoionization, laser desorption, chemical ionization, and 34 electrospray ionization.35 Note 3: Loss of an electron from a singly, doubly, etc. charged cation is called 36 second, third, etc. ionization.37 See also dissociation, ionization energy.38 rev[3]39 revGB-revPOC40

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1 ionization energy2 Ei

3 (SI unit J)4 Minimum energy required to remove an electron from an isolated molecular entity (in 5 its vibrational ground state) in the gaseous phase.6 Note 1: If the resulting molecular entity is in its vibrational ground state, the energy 7 is the "adiabatic ionization energy".8 Note 2: If the molecular entity produced possesses the vibrational energy determined 9 by the Franck-Condon principle (according to which the electron ejection takes place

10 without an accompanying change in molecular geometry), the energy is the "vertical 11 ionization energy".12 Note 3: The name ionization energy is preferred to the somewhat misleading earlier 13 name "ionization potential".14 See also ionization.15 GB[3]1617 ionizing power18 Tendency of a particular solvent to promote ionization of a solute.19 Note: The term has been used in both kinetic and thermodynamic contexts.20 See also Dimroth-Reichardt ET parameter, Grunwald-Winstein equation, Z-value.21 GB[3]2223 ipso attack24 Attachment of an entering group to a position in an aromatic compound already carrying 25 a substituent group other than hydrogen.26 Note: The entering group may displace that substituent group or may itself be 27 expelled or migrate to a different position in a subsequent step. The term "ipso-28 substitution" is not used, since it is synonymous with substitution.29 Example:

3031 where E+ is an electrophile and Z is a substituent other than hydrogen.32 See [262].33 See also cine-substitution, tele-substitution.34 GB[3]3536 isentropic37 See isoentropic.38

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1 isodesmic2 Property of a reaction (actual or hypothetical) in which the types of bonds that are made 3 in forming the products are the same as those that are broken in the reactants.4 Examples:

5 (i) PhCOOH + p-ClC6H4CO2– ⇄ PhCO2

– + p-ClC6H4COOH (1)

6 (ii) 2 CH3F ⇄ CH2F2 + CH4 (2)7 Note 1: Such processes have advantages for theoretical treatment.8 Note 2: The Hammett equation as applied to equilibria, as in (1), succeeds because 9 it deals with isodesmic processes.

10 Note 3: For the use of isodesmic processes in quantum chemistry, see [263].11 See [8].12 See also homodesmotic.13 rev[3]14 revGB1516 isoelectronic17 Having the same number of valence electrons and the same structure, i.e., number and 18 connectivity of atoms, but differing in some or all of the elements involved.19 Examples:20 (i) CO, N2, and NO+

21 (ii) CH2=C=O and CH2=N+=N–

22 GB[3]2324 isoentropic25 Isentropic 26 Property of a reaction series in which the individual reactions have the same standard 27 entropy or entropy of activation.28 GB[3]2930 isoequilibrium relationship31 Feature of a series of related substrates or of a single substrate under a series of 32 reaction conditions whereby the enthalpies and entropies of reaction can be correlated 33 by the equation3435 rH – rS = constant3637 Note: The parameter is called the isoequilibrium temperature.38 See [264,265].39 See also compensation effect, isokinetic relationship.

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1 GB[3]23 isokinetic relationship4 Feature of a series of related substrates or of a single substrate under a series of 5 reaction conditions whereby the enthalpies of activation and entropies of activation can 6 be correlated by the equation7 ‡H – ‡S = constant8 Note 1: The parameter is called the isokinetic temperature. At this temperature all 9 members of the series react at the same rate.

10 Note 2: Isokinetic relationships as established by direct correlation of ‡H with ‡S 11 are often spurious, and the calculated value of is meaningless, because errors in ‡H 12 lead to compensating errors in ‡S. Satisfactory methods of establishing such 13 relationships have been devised.14 See [264,265].15 See also compensation effect, isoequilibrium relationship, isoselective relationship.16 GB[3]1718 isolobal19 Feature of two molecular fragments for which the number, symmetry properties, 20 approximate energy, shape of the frontier orbitals, and number of electrons in them are 21 similar.22 Example:

23

CH

HMnH LL

L

L

L24 See also isoelectronic.25 rev[3]26 revGB-revPOC2728 isomer29 One of several species (or molecular entities) all of which have the same atomic 30 composition (molecular formula) but differ in their connectivity or stereochemistry and 31 hence have different physical and/or chemical properties.32 Note: Conformational isomers that interconvert by rapid rotation about single bonds 33 and configurations that interconvert by rapid pyramidal inversion are often not 34 considered as separate isomers.35 rev[3]36 revGB-revPOC37

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1 isomerization2 Chemical reaction in which the product is an isomer of the reactant.3 See also molecular rearrangement.4 rev[3]5 revGB-revPOC67 isosbestic point8 Wavelength (or frequency) at which two or more components in a mixture have the 9 same molar absorption coefficients.

10 Note 1: Isosbestic points are commonly met when electronic spectra are taken (a) 11 on a solution in which a chemical reaction is in progress (in which case the two 12 absorbing components concerned are a reactant and a product, A → B), or (b) on a 13 solution in which the two absorbing components are in equilibrium and their relative 14 proportions are controlled by the concentration of some other component, typically the 15 concentration of hydrogen ions, as in an acid-base indicator equilibrium.16 Note 2: The effect may also appear (c) in the spectra of a set of solutions of two or 17 more non-interacting components having the same total concentration.18 Note 3: In all these examples, A (and/or B) may be either a single chemical species 19 or a mixture of chemical species present in invariant proportion.20 Note 4: If absorption spectra of the types considered above intersect not at one or 21 more isosbestic points but over a progressively changing range of wavelengths, this is 22 prima facie evidence (a) for the formation of a reaction intermediate in substantial 23 concentration (A → C → B) or (b) for the involvement of a third absorbing species in 24 the equilibrium, e.g.,25 A ⇄ B + H+

aq ⇄ C + 2 H+aq

26 or (c) for some interaction of A and B, e.g.,27 A + B ⇄ C28 Note 5: Isobestic is a misspelling and is discouraged.29 rev[3]30 revGB-revPOC3132 isoselective relationship33 Relationship analogous to the isokinetic relationship, but applied to selectivity data of 34 reactions.35 Note: At the isoselective temperature, the selectivities of the series of reactions 36 following the relationship are identical, within experimental error.37 See [266].38 See also isoequilibrium relationship, isokinetic relationship.39 GB[3]40

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1 isotope effect2 Relative difference between, or the ratio of, the rate constants coefficients or equilibrium 3 constants of two reactions that differ only in the isotopic composition of one or more of 4 their otherwise chemically identical components.5 Note: The ratio kl/kh of rate constants (or Kl/Kh of equilibrium constants) for “light” and 6 “heavy” reactions is most often used in studies of chemical reaction mechanisms. 7 However, the opposite ratios are frequently used in environmental chemistry and 8 geochemistry; i.e., kh/kl or Kh/Kl. Furthermore the relative difference kh/kl – 1 or Kh/Kl – 9 1 (often expressed as the percentage deviation of the ratio from unity) is commonly

10 used to quantify isotopic fractionation.11 See kinetic isotope effect, equilibrium isotope effect.12 rev[3]13 revGB1415 isotope effect, equilibrium16 isotope effect, thermodynamic17 Effect of isotopic substitution on an equilibrium constant18 Note 1: For example, the effect of isotopic substitution in reactant A that participates 19 in the equilibrium:20 A + B ⇄ C21 is the ratio Kl/Kh of the equilibrium constant for the reaction in which A contains the light 22 isotope to that in which it contains the heavy isotope. The ratio can also be expressed 23 as the equilibrium constant for the isotopic exchange reaction:24 Al + Ch ⇄ Ah + Cl

25 in which reactants such as B that are not isotopically substituted do not appear.26 Note 2: The potential-energy surfaces of isotopic molecules are identical to a high 27 degree of approximation, so thermodynamic isotope effects can arise only from the 28 effect of isotopic mass on the nuclear motions of the reactants and products, and can 29 be expressed quantitatively in terms of nuclear partition functions:3031 Kl/Kh = [Ql(C)/Qh(C)]/ [Ql(A)/Qh(A)]3233 Note 3: Although the nuclear partition function is a product of the translational, 34 rotational, and vibrational partition functions, the isotope effect is usually determined 35 almost entirely by the last named, specifically by vibrational modes involving motion of 36 isotopically different atoms. In the case of light atoms (i.e., protium vs. deuterium or 37 tritium) at moderate temperatures, the isotope effect is dominated by zero-point energy 38 differences.39 See [267].40 See also fractionation factor.

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1 rev[3]2 revGB34 isotope effect, heavy-atom5 Isotope effect due to isotopes other than those of hydrogen.6 GB[3]78 isotope effect, intramolecular9 Kinetic isotope effect observed when a single substrate, in which the isotopic atoms

10 occupy equivalent reactive positions, reacts to produce a non-statistical distribution of 11 isotopologue products.1213 Example: PhCH2D + Br· → BrH + PhCHD· vs. BrD + PhCH2·1415 The intramolecular isotope effect kH/kD can be measured from the D content of product 16 (PhCH2Br + PhCHDBr), which is experimentally much easier than measuring the 17 intermolecular isotope effect kH/kD from the separate rates of reaction of PhCH3 and 18 PhCD3.19 rev[3]20 revGB-revPOC2122 isotope effect, inverse23 Kinetic isotope effect in which kl/kh < 1, i.e., the heavier substrate reacts more rapidly 24 than the lighter one, as opposed to the more usual "normal" isotope effect, in which kl/kh 25 > 1.26 Note: The isotope effect will be "normal" when the vibrational frequency differences 27 between the isotopic transition states are smaller than in the reactants. Conversely, an 28 inverse isotope effect can be taken as evidence for an increase in the force constants 29 on passing from the reactant to the transition state.30 GB[3]3132 isotope effect, kinetic33 Effect of isotopic substitution on a rate constant.34 For example in the reaction35 A + B → C36 the effect of isotopic substitution in reactant A is expressed as the ratio of rate 37 constants kl/kh, where the superscripts l and h represent reactions in which the 38 molecules A contain the light and heavy isotopes, respectively.39 Note 1: Within the framework of transition-state theory, where the reaction is 40 rewritten as

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1 A + B ⇄ [TS]‡ → C2 kl/kh can be regarded as if it were the equilibrium constant for an isotope exchange 3 reaction between the transition state [TS]‡ and the isotopically substituted reactant A, 4 and calculated from their vibrational frequencies as in the case of a thermodynamic 5 isotope effect (see thermodynamic (equilibrium) isotope effect).6 Note 2: Isotope effects like the above, involving a direct or indirect comparison of the 7 rates of reaction of isotopologues, are called "intermolecular", in contrast to 8 intramolecular isotope effects (see intramolecular isotope effect), in which a single 9 substrate reacts to produce a non-statistical distribution of isotopologue product

10 molecules.11 See [267,268].12 GB[3]1314 isotope effect, primary15 Kinetic isotope effect attributable to isotopic substitution of an atom to which a bond is 16 made or broken in the rate-limiting step or in a pre-equilibrium step of a reaction.17 Note: The corresponding isotope effect on the equilibrium constant of a reaction in 18 which one or more bonds to isotopic atoms are broken is called a primary equilibrium 19 isotope effect.20 See also secondary isotope effect.21 GB[3]2223 isotope effect, secondary24 Kinetic isotope effect that is attributable to isotopic substitution of an atom to which 25 bonds are neither made nor broken in the rate-limiting step or in a pre-equilibrium step 26 of a specified reaction.27 Note 1: The corresponding isotope effect on the equilibrium constant of such a 28 reaction is called a secondary equilibrium isotope effect.29 Note 2: Secondary isotope effects can be classified as , , etc., where the label 30 denotes the position of isotopic substitution relative to the reaction centre.31 Note 3: Although secondary isotope effects have been discussed in terms of 32 conventional electronic effects, e.g., induction, hyperconjugation, hybridization, such an 33 effect is not electronic but vibrational in origin.34 See [269].35 See also steric isotope effect.36 rev[3]37 revGB3839 isotope effect, solvent

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1 Kinetic or equilibrium isotope effect resulting from change in the isotopic composition 2 of the solvent.3 See also kinetic isotope effect, equilibrium isotope effect.4 GB[3]56 isotope effect, steric7 Secondary isotope effect attributed to the different vibrational amplitudes of 8 isotopologues.9 Note 1: For example, both the mean and mean-square amplitudes of vibrations

10 associated with C–H bonds are greater than those of C–D bonds. The greater effective 11 bulk of molecules containing the former may be manifested by a steric effect on a rate 12 or equilibrium constant. 13 Note 2: Ultimately the steric isotope effect arises from changes in vibrational 14 frequencies and zero-point energies, as normally do other isotope effects.15 rev[3]16 revGB1718 isotope effect, thermodynamic19 See isotope effect, equilibrium.20 GB[3]2122 isotope exchange23 Chemical reaction in which the reactant and product chemical species are chemically 24 identical but have different isotopic composition.25 Note: In such a reaction the isotope distribution tends towards equilibrium (as 26 expressed by fractionation factors) as a result of transfers of isotopically different atoms 27 or groups. For example,

28 GB[3]2930 isotopic perturbation, method of31 Measurement of the NMR shift difference due to the isotope effect on a fast 32 (degenerate) equilibrium between two species that are equivalent except for isotopic 33 substitution.34 Note: This method can distinguish a rapidly equilibrating mixture with time-averaged 35 symmetry from a single structure with higher symmetry.

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1 See [270,271,272,273].2 GB[3]34 isotopic scrambling5 Achievement of, or the process of achieving, a redistribution of isotopes within a 6 specified set of atoms in a chemical species or group of chemical species.7 Examples

89 (where * denotes the position of an isotopically different atom.)

10 See also fractionation factor.11 GB[3]1213 isotopologues14 Molecular entities that differ only in isotopic composition (number of isotopic 15 substitutions), e.g., CH4, CH3D, CH2D2, 16 Note: These are isotopic homologues. It is a misnomer to call them isotopomers, 17 because they are not isomers with the same atoms.18 rev[3]19 revGB2021 isotopomers22 Isomers having the same number of each isotopic atom but differing in their positions. 23 The term is a contraction of "isotopic isomer".24 Note: Isotopomers can be either constitutional isomers (e.g., CH2DCH=O and 25 CH3CD=O) or isotopic stereoisomers (e.g., (R)- and (S)-CH3CHDOH or (Z)- and (E)-26 CH3CH=CHD).27 See [11]. 28 GB[3]2930 Kamlet-Taft solvent parameters31 Quantitative measure of solvent polarity, based on the solvent’s hydrogen-bond donor, 32 hydrogen-bond acceptor, dipolarity/polarizability, and cohesive-pressure properties.33 See [274,275,276,277,278,279].

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1 See solvent parameter.2 rev[3]3 revGB-revPOC45 Kaptein-Closs rules6 Rules used to predict the sign of chemically induced dynamic nuclear polarization 7 (CIDNP) effects.8 See [280,281,282].9 GB

1011 Kekulé structure12 Representation of a molecular entity (usually aromatic) with fixed alternating single and 13 double bonds, in which interactions between multiple bonds are ignored.14 Example: For benzene the Kekulé structures are

1516 Note: The distinction among Lewis structure, Kekulé structure, and line formula is 17 now not generally observed, nor is the restriction to aromatic molecular entities.18 See also non-Kekulé structure.19 rev[3]20 revGB-revPOC2122 keto-enol tautomerization23 Interconversion of ketone and enol tautomers by hydron migration, accelerated by acid 24 or base catalysis.25 Example:

2627 See also tautomerization.2829 kinetic ambiguity30 deprecated31 See kinetic equivalence.

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1 GB23 kinetic control (of product composition)4 Conditions (including reaction times) that lead to reaction products in a proportion 5 governed by the relative rates of the parallel (forward) reactions by which those 6 products are formed, rather than by the equilibrium constants.7 See also thermodynamic control.8 GB[3]9

10 kinetic electrolyte effect11 kinetic ionic-strength effect12 General effect of an added electrolyte (i.e., other than, or in addition to, that due to its 13 possible involvement as a reactant or catalyst) on the rate constant of a reaction in 14 solution.15 Note 1: At low concentrations (when only long-range coulombic forces need to be 16 considered) the effect on a given reaction is determined only by the ionic strength of 17 the solution and not by the chemical identity of the ions. This concentration range is 18 roughly the same as the region of validity of the Debye-Hückel limiting law for activity 19 coefficients. At higher concentrations the effect of an added electrolyte depends also 20 on the chemical identity of the ions. Such specific actions can sometimes be interpreted 21 as the incursion of a reaction path involving an ion of the electrolyte as reactant or 22 catalyst, in which case the action is not properly to be regarded just as a kinetic 23 electrolyte effect. At higher concentrations the effect of an added electrolyte does not 24 necessarily involve a new reaction path, but merely the breakdown of the Debye-Hückel 25 law, whereby ionic activity coefficients vary with the ion.26 Note 2: Kinetic electrolyte effects are also called kinetic salt effects.27 Note 3: A kinetic electrolyte effect ascribable solely to the influence of the ionic 28 strength on activity coefficients of ionic reactants and transition states is called a primary 29 kinetic electrolyte effect. A kinetic electrolyte effect arising from the influence of the 30 ionic strength of the solution upon the pre-equilibrium concentration of an ionic species 31 that is involved in a subsequent rate-limiting step of a reaction is called a secondary 32 kinetic electrolyte effect. A common case is the secondary electrolyte effect on the 33 concentration of hydrogen ion (acting as catalyst) produced from the ionization of a 34 weak acid in a buffer solution. To eliminate the complication of kinetic electrolyte effects 35 in buffer solutions, it is advisable to maintain constant ionic strength.36 See [283].37 See also common-ion effect, order of reaction.38 GB[3]3940 kinetic equivalence

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1 Property of two reaction schemes that imply the same rate law.2 Example: Schemes (i) and (ii) for the formation of C from A under conditions that B 3 does not accumulate as a reaction intermediate:456789

10111213141516 Both equations for d[C]/dt are of the form

1718 where r and s are constants (sometimes called rate coefficients). The equations are 19 identical in their dependence on concentrations and do not distinguish whether HO– 20 catalyses the formation of B and its reversion to A, or is involved only in its further 21 transformation to C. The two schemes are therefore kinetically equivalent.22 GB[3]232425 Koppel-Palm solvent parameters26 Quantitative measure of solvent polarity, based on the solvent’s permittivity, refractive 27 index, basicity or nucleophilicity, and acidity or electrophilicity.28 See [284].29 See solvent parameter.30 rev[3]31 revGB3233 Kosower Z value34 See Z-value.35 GB[3]3637 labile38 Property of a chemical species that is relatively unstable and transient or reactive.

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1 Note: This term must not be used without explanation of the intended meaning.2 See also inert, persistent, reactivity, unreactive.3 GB[3]45 Laurence solvent parameters6 Quantitative measures of solvent polarity, based on the solvent’s dispersion/induction 7 interactions, electrostatic interactions between permanent multipoles, solute Lewis 8 base/solvent Lewis acid interactions, and solute HBD/solvent HBA interactions.9 See [285].

1011 least nuclear motion, principle of (hypothesis of least motion)12 Hypothesis that, for given reactants, the reactions involving the smallest change in 13 nuclear positions will have the lowest energy of activation.14 Note: The basis for this hypothesis is that the energy of a structural deformation 15 leading toward reaction is proportional to the sum of the squares of the changes in 16 nuclear positions, which holds only for small deformations and is therefore not always 17 valid.18 See [8,286].19 rev[3]20 revGB2122 leaving group23 Species (charged or uncharged) that carries away the bonding electron pair when it 24 becomes detached from another fragment in the residual part of the substrate.25 Example 1: In the heterolytic solvolysis of (bromomethyl)benzene (benzyl bromide) 26 in acetic acid2728 PhCH2Br + AcOH → PhCH2OAc + HBr2930 the leaving group is Br–.31 Example 2: In the reaction3233 MeS– + PhCH2N+Me3 → MeSCH2Ph + NMe33435 the leaving group is NMe3.36 Note: The historical term “leaving group” is ambiguous, because in the heterolysis of 37 R–X, both R+ and X- are fragments that leave from each other. For that reason, X– (as 38 well as Br– and NMe3 in examples 1 and 2) can unambiguously be called nucleofuges, 39 whereas R+ is an electrofuge.40 See also electrofuge, entering group, nucleofuge.41 rev[3]

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1 revGB-revPOC23 Leffler's relation4 Leffler’s assumption5 In a series of elementary reactions the changes in Gibbs activation energies are often 6 found to be proportional to the changes in Gibbs energies for the overall reaction.78 ‡G = rG0

910 This relation was interpreted in terms of the simple assumption that a small change in 11 any transition-state property P‡ is a linear combination of changes in reactant- and 12 product-state properties, PR and PP.1314 P‡ = PP + (1 –) PR

1516 Within the limits of this assumption, the parameter is an approximate measure of the 17 fractional displacement of the transition state along the minimum-energy reaction path 18 from reactants to products.19 See [109].20 Note: There are many exceptions to the validity of Leffler’s assumption that is a 21 measure of the position of the transition state.22 See [287].23 See Hammond postulate.24 rev[3]25 revGB-revPOC2627 left-to-right convention28 Arrangement of the structural formulae of the reactants so that the bonds to be made 29 or broken form a linear array in which the electron pushing proceeds from left to right.30 See [288].31 GB[3]3233 levelling effect34 Tendency of a solvent to make all Brønsted acids whose acidity exceeds a certain value 35 appear equally acidic.36 Note 1: This phenomenon is due to the complete transfer of a hydron to a Brønsted-37 basic solvent from a dissolved acid stronger than the conjugate acid of the solvent. The 38 only acid present to any significant extent in all such solutions is the lyonium ion.

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1 Note 2: For example, the solvent water has a levelling effect on the acidities of HClO4, 2 HCl, and HI. Aqueous solutions of these acids at the same (moderately low) 3 concentrations have the same acidities.4 Note 3: A corresponding levelling effect applies to strong bases in protogenic 5 solvents.6 GB[3]78 Lewis acid9 Molecular entity (and the corresponding chemical species) that is an electron–pair

10 acceptor and therefore able to react with a Lewis base to form a Lewis adduct by 11 sharing the electron pair furnished by the Lewis base.12 Example:1314 Me3B (Lewis acid) + :NH3 (Lewis base) → Me3B––N+H3 (Lewis adduct)1516 See also coordination.17 GB[3]1819 Lewis acidity20 Thermodynamic tendency of a substrate to act as a Lewis acid. 21 Note 1: This property is defined quantitatively by the equilibrium constant or Gibbs 22 energy for Lewis adduct formation of a series of Lewis acids with a common reference 23 Lewis base.24 Note 2. An alternative measure of Lewis acidity in the gas phase is the enthalpy of 25 Lewis adduct formation for that Lewis acid with a common reference Lewis base.26 See also electrophilicity, Lewis basicity.27 rev[3]28 revGB2930 Lewis adduct31 Adduct formed between a Lewis acid and a Lewis base.32 GB[3]3334 Lewis base35 Molecular entity (and the corresponding chemical species) able to provide a pair of 36 electrons and thus capable of coordination to a Lewis acid, thereby producing a Lewis 37 adduct.38 GB[3]3940 Lewis basicity

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1 Thermodynamic tendency of a substance to act as a Lewis base.2 Note 1: This property is defined quantitatively by the equilibrium constant or Gibbs 3 energy of Lewis adduct formation for that Lewis base with a common reference Lewis 4 acid.5 Note 2: An alternative measure of Lewis basicity in the gas phase is the enthalpy of 6 Lewis adduct formation for that Lewis base with a common reference Lewis acid.7 See also donicity, Lewis acidity, nucleophilicity, proton affinity.8 rev[3]9 revGB

1011 Lewis structure12 electron-dot structure, Lewis formula13 Representation of molecular structure in which (a) nonbonded valence electrons are 14 shown as dots placed adjacent to the atoms with which they are associated and in 15 which (b) a pair of bonding valence electrons in a covalent bond is shown as two dots 16 between the bonded atoms, and in which (c) formal charges (e.g., +, –, 2+) are attached 17 to atoms to indicate the difference between the nuclear charge (atomic number) and 18 the total number of electrons associated with that atom, on the formal basis that bonding 19 electrons are shared equally between atoms they join.20 Examples:

2122 Note 1: A double bond is represented by two pairs of dots, and a triple bond by three 23 pairs, as in the last example above.24 Note 2: Bonding pairs of electrons are usually denoted by lines, representing 25 covalent bonds, as in line formulas, rather than as a pair of dots, and the lone pairs are 26 sometimes omitted.27 Examples:

2829 Note 3: The distinction among Lewis structure, Kekulé structure, and line formula is 30 now not generally observed.31 rev[3]32 revGB3334 lifetime (mean lifetime)35 36 Time needed for the concentration of a chemical species that decays in a first-order 37 process to decrease to 1/e of its original value. i.e., c(t = ) = c(t = 0)/e.

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1 Note 1: Statistically, it represents the mean life expectancy of the species.2 Note 2: Mathematically: = 1/k = 1/(iki) where ki is the first-order rate constant for 3 the i-th decay process of the species.4 Note 3: Lifetime is sometimes applied to processes that are not first-order. However, 5 in such cases the lifetime depends on the initial concentration of the entity or of a 6 quencher and, therefore, only an initial lifetime can be defined. In this case, it should be 7 called decay time.8 See [9].9 See also chemical relaxation, half-life, rate of reaction.

10 GB[3]1112 ligand13 Atom or group bound to a central atom in a polyatomic molecular entity (if it is possible 14 to indicate such a central atom).15 See [29].16 Note 1:The term is generally used in connection with metallic central atoms.17 Note 2: In biochemistry a part of a polyatomic molecular entity may be considered 18 central, and atoms, groups, or molecules bound to that part are considered ligands.19 See [289], p. 335.20 GB[3]2122 line formula23 Two-dimensional representation of molecular entities in which atoms are shown joined 24 by lines representing single bonds (or multiple lines for multiple bonds), without any 25 indication or implication concerning the spatial direction of bonds.26 Examples: methanol, ethene

2728 See also condensed formula, Kekulé formula, Lewis formula, skeletal formula. 29 rev[3]30 revGB-revPOC3132 linear free-energy relation33 linear Gibbs-energy relation34 GB[3]3536 linear Gibbs-energy relation37 linear free-energy relation (LFER)

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1 Linear correlation between the logarithm of a rate constant or equilibrium constant for 2 a series of reactions and the logarithm of the rate constant or equilibrium constant for a 3 related series of reactions.4 Typical examples of such relations are the Brønsted relation and the Hammett 5 equation (see also -value).6 Note: The name arises because the logarithm of the value of an equilibrium constant 7 (at constant temperature and pressure) is proportional to a standard Gibbs energy (free 8 energy) change, and the logarithm of the value of a rate constant is a linear function of 9 the Gibbs energy (free energy) of activation.

10 GB[3]1112 linear solvation-energy relationship (LSER)13 Application of solvent parameters in the form of a single- or multi-parameter equation 14 expressing the solvent effect on a given property: e.g., rate of reaction, equilibrium 15 constant, spectroscopic shift.16 Note: The solvent effect may be estimated as a linear combination of elementary 17 effects on a given property P, relative to the property P0 in the reference solvent:1819 P – P0 = a A + b B + p DP…2021 where A, B, DP, etc. are acidity, basicity, dipolarity, etc. parameters, and a, b, p… are 22 the sensitivity of the property to each effect.23 See Catalán solvent parameters, Dimroth-Reichardt ET parameter, Kamlet-Taft 24 solvent parameters, Koppel-Palm solvent parameters, Laurence solvent parameters, 25 Z-value.26 rev[3]27 revGB2829 line-shape analysis30 Method for determination of rate constants for dynamic chemical exchange from the 31 shapes of spectroscopic signals, most often used in nuclear magnetic resonance 32 spectroscopy.33 GB[3]3435 Lineweaver-Burk plot36 See Michaelis-Menten kinetics.37 GB[3]3839 lipophilic

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1 Feature of molecular entities (or parts of molecular entities) that have a tendency to 2 dissolve in fat-like solvents (e.g., hydrocarbons).3 See also hydrophilic, hydrophobic interaction.4 rev[3]5 GB67 London forces8 dispersion forces 9 Attractive forces between molecules due to their mutual polarizability.

10 Note: London forces are the principal components of the forces between nonpolar 11 molecules.12 See [290].13 See also van der Waals forces.14 GB[3]1516 lone (electron) pair (nonbonding electron pair)17 Two spin-paired electrons localized in the valence shell on a single atom.18 Note: In structural formulas lone pairs should be designated as two dots.19 See also Lewis structure.20 rev[3]21 GB2223 LUMO24 Acronym for Lowest Unoccupied Molecular Orbital25 See frontier orbitals.26 rev[3]27 GB2829 lyate ion30 Anion produced by hydron (proton, deuteron, triton) removal from a solvent molecule.31 Example: the hydroxide ion is the lyate ion of water.32 GB[3]3334 lyonium ion35 Cation produced by hydronation (protonation, deuteronation, tritonation) of a solvent 36 molecule.37 Example: CH3OH2

+ is the lyonium ion of methanol.38 See also onium ion.39 GB[3]40

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123 macroscopic diffusion control4 See mixing control.5 GB[3]67 magnetic equivalence8 Property of nuclei that have the same resonance frequency in nuclear magnetic 9 resonance spectroscopy and also identical spin-spin interactions with each nucleus of

10 a neighbouring group.11 Note 1: The spin-spin interaction between magnetically equivalent nuclei is not 12 manifested in the spectrum, and has no effect on the multiplicity of the respective NMR 13 signals.14 Note 2: Magnetically equivalent nuclei are necessarily chemically equivalent, but the 15 reverse is not necessarily true.16 GB[3]1718 magnetization transfer19 NMR method for determining kinetics of chemical exchange by perturbing the 20 magnetization of nuclei in a particular site or sites and following the rate at which 21 magnetic equilibrium is restored.22 Note: The most common perturbations are saturation and inversion, and the 23 corresponding techniques are often called "saturation transfer" and "selective inversion-24 recovery".25 See also saturation transfer.26 GB[3]2728 Marcus equation29 General expression that correlates the Gibbs energy of activation (‡G) with the Gibbs 30 energy of the reaction (rGº)3132 ‡G = (/4)(1 + rGº/)2 = ‡Go + 1/2 rGº + (rGº)2/(16 ‡Go)3334 where is the reorganization energy and ‡Go is the intrinsic barrier, with = 4‡Go.35 Note: Originally developed for outer-sphere electron transfer reactions, the Marcus 36 equation applies to many atom- and group-transfer reactions. The Marcus equation 37 captures earlier ideas that reaction thermodynamics can influence reaction barriers: 38 e.g., the Brønsted relation (1926), the Bell-Evans-Polanyi principle (1936-38), Leffler’s 39 relation (1953), and the Hammond postulate (1955) [227]. It also implies that changes 40 in intrinsic barriers may dominate over changes of reaction Gibbs energies and thus

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1 account for the fact that reaction rates may not be controlled by the relative 2 thermodynamic stabilities of the products.3 See [291,292,293,294,295,296]. 4 rev[3]56 Markovnikov (Markownikoff) rule7 Statement of the common mechanistic observation that in electrophilic addition 8 reactions the more electropositive atom (or part) of a polar molecule becomes attached 9 to the carbon bearing more hydrogens.

10 Note 1: This rule was originally formulated by Markownikoff (Markovnikov) as "In the 11 addition of hydrogen halides to [unsaturated] hydrocarbons, the halogen atom becomes 12 attached to the carbon bearing the lesser number of hydrogen atoms".13 Note 2: This rule can be rationalized as the addition of the more electropositive atom 14 (or part) of the polar molecule to the end of the multiple bond that would result in the 15 more stable carbenium ion (regardless of whether the carbenium ion is a stable 16 intermediate or a transient structure along the minimum-energy reaction path).17 Note 3: Addition in the opposite sense, as in radical addition reactions, is commonly 18 called anti-Markovnikov addition.19 See [297].20 GB[3]2122 mass action, law of23 Statement that the velocity of a reaction depends on the active mass, i.e., the 24 concentrations of the reactants. 25 Example: for an association reaction (1) and its reverse (2)2627 A + B AB (1)28 AB A + B (2)2930 the forward velocity is v1 = k1 [A][B], with k1 the rate constant for the association reaction. 31 For the dissociation reaction 2 the velocity is v2 = k2 [AB]. This is valid only for 32 elementary reactions. Furthermore, the law of mass action states that, when a 33 reversible chemical reaction reaches equilibrium at a given temperature, the forward 34 rate is the same as the backward rate. Therefore, the concentrations of the chemicals 35 involved bear a constant relation to each other, described by the equilibrium constant, 36 i.e., for 3738 A + B AB 39

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1 in equilibrium, v1 = k1 [A][B] = v2 = k2 [AB] and one form of the equilibrium constant for 2 the above chemical reaction is the ratio 3

4 Kc = = [AB][A][B]

𝑘1

𝑘2

56 Note: First recognized in 1864 as the kinetic law of mass action by Guldberg and 7 Waage, who first introduced the concept of dynamic equilibrium, but incorrectly 8 assumed that the rates could be deduced from the stoichiometric equation [298]. Only 9 after the work of Horstmann [299] and van’t Hoff [300] a mathematical derivation of the

10 reaction rates considering the order of the reaction involved was correctly made. 11 See also [301,302,303].12 rev[3]13 revGB-revPOC1415 matrix isolation16 Technique for preparation of a reactive or unstable species by dilution in an inert solid 17 matrix (argon, nitrogen, etc.), usually condensed on a window or in an optical cell at low 18 temperature, to preserve its structure for identification by spectroscopic or other means.19 See [304].20 GB[3]2122 Mayr-Patz equation23 Rate constants for the reactions of sp2-hybridized electrophiles with nucleophiles can 24 be expressed by the correlation2526 lg [k/ (dm3 mol-1 s-1)] = sN(E + N),2728 where E is the nucleophile-independent electrophilicity parameter, N is the electrophile-29 independent nucleophilicity parameter, and sN is the electrophile-independent 30 nucleophile-specific susceptibility parameter. 31 Note 1: This equation is equivalent to the conventional linear Gibbs energy 32 relationship lg k = Nu + sNE, where Nu = sNN. The use of N is preferred, because it 33 provides an approximate ranking of relative reactivities of nucleophiles.34 Note 2: The correlation should not be applied to reactions with bulky electrophiles, 35 where steric effects cannot be neglected. Because of the way of parametrization, the 36 correlation is applicable only if one or both reaction centres are carbon.37 Note 3: As the E parameters of the reference electrophiles are defined as solvent-38 independent, all solvent effects are shifted into the parameters N and sN.39 Note 4: The equation transforms into the Ritchie equation for sN = 1.

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1 Note 5: Applications to SN2-type reactions are possible if an electrophile-specific 2 susceptibility parameter is introduced.3 See [305,306,307,308].4 See also Ritchie equation, Swain-Scott equation.5 The Mayr scale is available at http://www.cup.lmu.de/oc/mayr/DBintro.htm67 mechanism8 Detailed description of the process leading from the reactants to the products of a 9 reaction, including a characterization as complete as possible of the composition,

10 structure, energy and other properties of reaction intermediates, products, and 11 transition states.12 Note 1: An acceptable mechanism of a specified reaction (and there may be a 13 number of such alternative mechanisms not excluded by the evidence) must be 14 consistent with the reaction stoichiometry, the rate law, and with all other available 15 experimental data, such as the stereochemical course of the reaction. 16 Note 2: Inferences concerning the electronic motions that dynamically interconvert 17 successive species along the reaction path (as represented by curved arrows, for 18 example) are often included in the description of a mechanism.19 rev[3]20 revGB-revPOC2122 mechanism-based inhibition (suicide inhibition)23 Irreversible inhibition of an enzyme by formation of covalent bond(s) between the 24 enzyme and the inhibitor, which is a substrate analogue that is converted by the enzyme 25 into a species that reacts with the enzyme.26 rev[3]27 revGB-revPOC2829 medium30 Phase (and composition of the phase) in which chemical species and their reactions 31 are studied.32 GB[3]3334 Meisenheimer adduct (Jackson-Meisenheimer adduct)35 Lewis adduct formed in nucleophilic aromatic substitution from a nucleophile (Lewis 36 base) and an aromatic or heteroaromatic compound,

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12 Note 1: In cases where the substrate lacks electron-withdrawing groups, and 3 depending also on the nucleophile, the Meisenheimer adduct is not a minimum on the 4 potential-energy surface but a transition state.5 See [309,310,311,312]. 6 Note 2: Analogous cationic adducts, such as7

89

10 which are intermediates in electrophilic aromatic substitution reactions, are instead 11 called Wheland intermediates or -adducts (or the discouraged term -complexes).12 See [313,314].13 rev[3]14 revGB-revPOC1516 melting point (corrected/uncorrected)17 Temperature at which liquid and solid phases coexist in equilibrium, as measured with 18 a thermometer whose reading was corrected (or not) for the emergent stem that is in 19 ambient air.20 Note: In current usage the qualification often means that the thermometer was/(was 21 not) calibrated or that its accuracy was/(was not verified). This usage is inappropriate 22 and should be abandoned.23 GB[3]2425 mesolytic cleavage26 Cleavage of a bond in a radical ion whereby a radical and an ion are formed. The term 27 reflects the mechanistic duality of the process, which can be viewed as homolytic or 28 heterolytic depending on how the electrons are assigned to the fragments.29 See [315,316].30 GB[3]3132 mesomerism, mesomeric structure33 obsolete34 Resonance, resonance form35 rev[3]36 revGB3738 mesophase

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1 Phase of a liquid crystalline compound between the crystalline and the isotropic liquid 2 phase.3 See [317].4 difGB[3]56 metastable (chemical species)7 deprecated8 Transient (chemical species).9 GB[3]

1011 metathesis12 Process formally involving the redistribution of fragments between similar chemical 13 species so that the bonds to those fragments in the products are identical (or closely 14 similar) to those in the reactants.15 Example:

1617 Note: The term has its origin in inorganic chemistry, with a different meaning, but that 18 older usage is not applicable in physical organic chemistry.19 GB[3]2021 micellar catalysis22 Acceleration of a chemical reaction in solution by the addition of a surfactant at a 23 concentration higher than its critical micelle concentration so that the reaction can 24 proceed in the environment of surfactant aggregates (micelles).25 Note 1: Rate enhancements may be due to a higher concentration of the reactants 26 in that environment, or to a more favourable orientation and solvation of the species, or 27 to enhanced rate constants in the micellar pseudophase of the surfactant aggregate.28 Note 2: Micelle formation can also lead to a decreased reaction rate.29 See [318].30 See also catalyst.31 GB[3]3233 micelle34 Aggregate of 1- to 1000-nm diameter formed by surfactants in solution, which exists in 35 equilibrium with the molecules or ions from which it is formed.36 See [135].37 See also inverted micelle.

RCH CHR

RCH CHR

RCH

RCH

RCH

RCH

catalyst

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1 rev[3]2 revGB-revPOC34 Michaelis-Menten kinetics5 Appearance of saturation behavior in the dependence of the initial rate of reaction v0 6 on the initial concentration [S]0 of a substrate when it is present in large excess over 7 the concentration of an enzyme or other catalyst (or reagent) E, following the equation,89 v0 = Vmax [S]0 /(KM + [S]0),

1011 where v0 is the observed initial rate, Vmax is its limiting value at substrate saturation (i.e., 12 when [S]0 >> KM, so that all enzyme is bound to substrate), and KM is the substrate 13 concentration at which v0 = Vmax/2.14 Note 1: Empirical definition, applying to any reaction that follows an equation of this 15 general form.16 Note 2: Often only initial rates are measured, at low conversion, so that [S]0 can be 17 considered as time-independent but varied from run to run.18 Note 3: The parameters Vmax and KM (the Michaelis constant) can be evaluated from 19 slope and intercept of a linear plot of 1/v0 against 1/[S]0 (Lineweaver-Burk plot) or from 20 slope and intercept of a linear plot of [S]0/v0 against [S]0 (Eadie-Hofstee plot), but a 21 nonlinear fit, which is readily performed with modern software, is preferable.22 Note 4: This equation is also applicable to the condition where E is present in large 23 excess, in which case [E] appears in the equation instead of [S]0.24 Note 5: The term has been used to describe reactions that proceed according to the 25 scheme

26

E + S ES Products + Ek1

k-1

kcat

2728 in which case KM = (k–1 + kcat)/k1 (Briggs-Haldane conditions). It has more usually been 29 applied only to the special case in which k–1 >> kcat and KM = k–1/k1; in this case KM is a 30 true dissociation constant (Michaelis-Menten conditions).31 See [319,320].32 GB[3]3334 microscopic diffusion control (encounter control)35 Observable consequence of the limitation that the rate of a bimolecular chemical 36 reaction in a homogeneous medium cannot exceed the rate of encounter of the reacting 37 molecular entities.38 Note: The maximum rate constant is usually in the range 109 to 1010 dm3 mol–1 s–1 in 39 common solvents at room temperature.

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1 See also mixing control.2 rev[3]3 revGB45 microscopic reversibility, principle of6 In a reversible reaction the mechanism in one direction is exactly the reverse of the 7 mechanism in the other direction.8 See also chemical reaction, detailed balancing.9 revGB[3]

1011 migration12 Transfer (usually intramolecular) of an atom or group during the course of a molecular 13 rearrangement.14 Example (of a hydrogen migration):1516 RCH2CH=CH2 → RCH=CHCH3

17 rev[3]18 revGB1920 migratory aptitude21 Tendency of a group to participate in a molecular rearrangement, relative to that of 22 another group, often in the same molecule.23 Example: In the Baeyer-Villiger rearrangement of PhCOCH3, via intermediate 24 PhC(OH)(OOCOAr)CH3, the major product is CH3COOPh, by phenyl migration, rather 25 than PhCOOCH3.26 Note: In nucleophilic rearrangements (migration to an electron-deficient centre) the 27 migratory aptitude of a group is loosely related to its capacity to stabilize a partial 28 positive charge, but exceptions are known, and the position of hydrogen in the series is 29 often unpredictable.30 rev[3]31 revGB3233 migratory insertion34 Reaction that involves the migration of a group to another position on a metal centre, 35 with insertion of that group into the bond between the metal and the group that is in that 36 other position. 37 Examples:38 CO insertion39

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123 Ziegler-Natta reaction4

5LnM

RLnM

RLnM

CH2CH2RCH2 CH2 migratory

insertion

67 Note: On repetition of the reaction the substituent alternates between the two 8 positions.9 rev[3]

10 revGB1112 minimum structural change, principle of13 Principle that a chemical reaction is expected to occur with a minimum of bond changes 14 or with a minimum redistribution of electrons (although more complex reaction 15 cascades are also possible).16 Example:17

18

Cl Cl

1920 where the transfer of Cl is accompanied by migration of only one carbon, rather than a 21 more extensive molecular rearrangement involving migration of three methyls.22 See [286]. 23 See also least nuclear motion, principle of.24 rev[3]25 revGB2627 minimum-energy reaction path (MERP)28 Path of steepest-descent from a saddle point on a potential-energy surface in each 29 direction towards adjacent energy minima; equivalent to the energetically easiest route 30 from reactants to products.31 See intrinsic reaction coordinate.32 See [8].

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1 GB23 mixing control4 Experimental limitation of the rate of reaction in solution by the rate of mixing of 5 solutions of the two reactants.6 Note 1: Mixing control can occur even when the reaction rate constant is several 7 orders of magnitude less than that for an encounter-controlled reaction.8 Note 2: Analogous (and more important) effects of the limitation of reaction rates by 9 the rate of mixing are encountered in heterogeneous (solid/liquid, solid/gas, liquid/gas)

10 systems.11 See [321].12 See also microscopic diffusion control, stopped flow.13 GB[3]14151617 Möbius aromaticity18 Feature of a monocyclic array of orbitals in which there is a single out-of-phase 19 overlap (or, more generally, an odd number of out-of-phase overlaps), whereby the 20 pattern of aromatic character is opposite to Hückel systems; with 4n electrons it is 21 stabilized (aromatic), whereas with 4n + 2 it is destabilized (antiaromatic).22 Note 1: The name is derived from the topological analogy of such an arrangement of 23 orbitals to a Möbius strip.24 Note 2: The concept has been applied to transition states of pericyclic reactions.Note 25 3: In the electronically excited state 4n + 2 Möbius -electron systems are stabilized, 26 and 4n systems are destabilized.27 Note 4: A few examples of ground-state Möbius systems are known [322,323].28 See [324,325]. 29 See also aromatic, Hückel (4n + 2) rule.30 rev[3]31 revGB3233 molecular entity34 Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical 35 ion, complex, conformer, etc., identifiable as a separately distinguishable entity.36 Note 1: Molecular entity is used in this glossary as a general term for singular entities, 37 irrespective of their nature, while chemical species stands for sets or ensembles of 38 molecular entities. Note that the name of a substance may refer to the molecular entity 39 or to the chemical species, e.g., methane, may mean either a single molecule of CH4

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1 (molecular entity) or an ensemble of such species, specified or not (chemical species), 2 participating in a reaction.3 Note 2: The degree of precision necessary to describe a molecular entity depends 4 on the context. For example "hydrogen molecule" is an adequate definition of a certain 5 molecular entity for some purposes, whereas for others it may be necessary to 6 distinguish the electronic state and/or vibrational state and/or nuclear spin, etc. of the 7 molecule.8 GB[3]9

10 molecular formula11 List of the elements in a chemical species or molecular entity, with subscripts indicating 12 how many atoms of each element are included.13 Note: In organic chemistry C and H are listed first, then the other elements in 14 alphabetical order.15 See [29].16 See also empirical formula.171819 molecular mechanics (MM) (empirical force-field calculation)20 Computational method intended to give estimates of structures and energies for 21 molecules. 22 Note: Even though such calculations can be made with either classical or quantum 23 mechanics (or both), the term molecular mechanics is widely understood as a classical 24 mechanics method that does not explicitly describe the electronic structure of the 25 molecular entities. It is based on the assumption of preferred bond lengths and angles, 26 deviations from which lead to strain, and the existence of torsional interactions and 27 attractive and repulsive van der Waals and Coulombic forces between non-bonded 28 atoms, all of which are parametrized to fit experimental properties such as energies or 29 structures. In contrast, in the quantum mechanical implementation no such 30 assumptions/parameters are needed.31 See [8,326].32 rev[3]33 revGB3435 molecular metal36 Non-metallic material whose properties, such as conductivity, resemble those of metals, 37 usually following oxidative doping.38 Example: polyacetylene following oxidative doping with iodine.39 GB[3]40

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1 molecular orbital2 One-electron wavefunction describing a single electron moving in the field provided by 3 the nuclei and all other electrons of a molecular entity of more than one atom.4 Note 1: Molecular orbitals describing valence electrons are often delocalized over 5 several atoms of a molecule. They are conveniently expressed as linear combinations 6 of atomic orbitals. They can be described as two-centre, multi-centre, etc. in terms of 7 the number of nuclei (or "centres") encompassed. 8 Note 2: For molecules with a plane of symmetry, a molecular orbital can be classed 9 as sigma () or pi (), depending on whether the orbital is symmetric or antisymmetric

10 with respect to reflection in that plane.11 See atomic orbital, orbital.12 See [8].13 rev[3]14 revGB1516 molecular rearrangement17 Reaction of a molecular entity that involves a change of connectivity.18 Note 1: The simplest type of rearrangement is an intramolecular reaction in which 19 the product is isomeric with the reactant (intramolecular isomerization). An example is 20 the first step of the Claisen rearrangement.21

2223 Note 2: The definition of molecular rearrangements includes reactions in which there 24 is a migration of an atom or bond (unexpected on the basis of the principle of minimum 25 structural change), as in the reaction2627 CH3CH2CH2Br + AgOAc → (CH3)2CHOAc + AgBr2829 where the rearrangement step can formally be represented as the "1,2-shift" of hydride 30 between adjacent carbon atoms in a carbocation3132 CH3CH2CH2

+ → (CH3)2CH+

3334 Note 3: Such migrations also occur in radicals, e.g.,3536 PhC(CH3)2–CH2· → ·C(CH3)2CH2Ph (neophyl rearrangement)

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12 Note 4: The definition also includes reactions in which an entering group takes up a 3 different position from the leaving group, with accompanying bond migration, such as 4 in the "allylic rearrangement":56 (CH3)2C=CHCH2Br + HO– → (CH3)2C(OH)CH=CH2 + Br–

78 Note 5: A distinction can be made between intramolecular rearrangements (or "true" 9 molecular rearrangements) and intermolecular rearrangements (or apparent

10 rearrangements). In the former case the atoms and groups that are common to a 11 reactant and a product never separate into independent fragments during the 12 rearrangement stage, whereas in an intermolecular rearrangement a migrating group 13 becomes completely free from the parent molecule and is re-attached to a different 14 position in a subsequent step, as in the Orton reaction:1516 C6H5N(Cl)COCH3 + HCl → C6H5NHCOCH3 + Cl2 17 → o- and p-ClC6H4NHCOCH3 + HCl18 See [327].19 rev[3]20 revGB2122 molecular recognition23 Attraction between specific molecules through noncovalent interactions that often 24 exhibit electrostatic and stereochemical complementarity between the partners25 Note: The partners are usually designated as host and guest, where the host 26 recognizes and binds the guest with high selectivity over other molecules of similar size 27 and shape. 2829 molecularity30 Number of reactant molecular entities that are involved in the "microscopic chemical 31 event" constituting an elementary reaction.32 Note 1: For reactions in solution this number is always taken to exclude molecular 33 entities that form part of the medium and which are involved solely by virtue of their 34 solvation of solutes.35 Note 2: A reaction with a molecularity of one is called "unimolecular", one with a 36 molecularity of two "bimolecular", and of three "termolecular".37 See also chemical reaction, order of reaction.38 GB[3]3940 molecule

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1 An electrically neutral entity consisting of more than one atom.2 Note: Rigorously, a molecule must correspond to a depression on the potential-3 energy surface that is deep enough to confine at least one vibrational state.4 See also molecular entity.5 GB[3]67 More O'Ferrall - Jencks diagram8 Conceptual visualization of the potential-energy surface for a reacting system, as a 9 function of two coordinates, usually bond lengths or bond orders.

10 Note 1: The diagram is useful for analyzing structural effects on transition-state 11 geometry and energy.12 Note 2: According to the Hammond postulate, stabilization of the products relative to 13 the reactants (an effect parallel to the minimum-energy reaction path, MERP) shifts the 14 transition state away from the product geometry, whereas destabilization of the 15 products shifts the transition state towards the product geometry. As first noted by 16 Thornton, stabilization of a structure located off the assumed MERP in a direction 17 perpendicular to it shifts the transition state toward that more stabilized geometry (a 18 perpendicular effect); destabilization shifts the transition state in the opposite direction.19

20 2122 Figure. More O'Ferrall-Jencks diagram for β-elimination reaction, with reactants, 23 products, and possible intermediates at the four corners.24

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1 Case 1: In a concerted elimination the transition state (‡1) is a saddle point near the 2 centre of the diagram, and the assumed MERP follows close to the diagonal.3 Case 2: If the carbanion intermediate is stabilized, the transition state shifts toward 4 that intermediate, to a transition state (‡2) in which the C–H bond is more extensively 5 broken and the C–X bond is more intact. Conversely, if the carbocation intermediate is 6 stabilized, the transition state shifts towards the top-left corner of the Figure, with less 7 C–H bond breaking and more C–X bond making8 Case 3: If leaving group X– is stabilised, energy decreases along the dotted vertical 9 arrow. It follows that in the resulting transition state (‡3) the C–H bond is more intact but

10 there is little change in the C–X bond, the shift of the transition state is the resultant of 11 a parallel (Hammond) component away from the top-right corner and a perpendicular 12 (anti-Hammond) component towards the top-left corner, yielding a transition state (‡3) 13 with less C–H bond breaking but approximately no change in the extent of C–X bond 14 making. 15 See [229,328,329,330,331].16 See also Hammond Postulate, anti-Hammond effect.17 rev[3]18 revGB1920 Morse potential21 V(r)22 (unit J)23 Empirical function relating the potential energy of a molecule to the interatomic distance 24 r accounting for the anharmonicity of bond stretching:2526 V(r) = De{1 – exp[–a(r–re)]}2

2728 where De is the bond-dissociation energy, re is the equilibrium bond length, and a is a 29 parameter characteristic of a given molecule.30 See [8].31 GB3233 mu34 µ35 (1) Symbol used to designate (as a prefix) a ligand that bridges two or more atoms.36 Note: If there are more than two atoms being bridged, μ carries a subscript to denote 37 the number of atoms bridged.38 (2) Symbol used to designate dipole moment as well as many other terms in physics 39 and physical chemistry.40

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1 multi-centre bond2 Bond in which an electron pair is shared among three or more atomic centres.3 Note 1: This may be needed when the representation of a molecular entity solely by 4 localized two-electron two-centre bonds is unsatisfactory, or when there are not enough 5 electrons to allow one electron pair shared between two adjacent atoms.6 Note 2: This is restricted to σ bonds and does not apply to species with delocalized 7 electrons.8 Examples include the three-centre bonds in diborane B2H6 and in bridged 9 carbocations.

10 rev[3]11 revGB-revPOC1213 multident14 multidentate15 See ambident.16 GB[3]1718 nanomaterial19 Substance whose particles are in the size range of 1 to 100 nm.20 Note: This may have chemical properties different from those of the corresponding 21 bulk material.2223 narcissistic reaction24 Chemical reaction that can be described as the automerization or enantiomerization of 25 a reactant into its mirror image (regardless of whether the reactant is chiral). 26 Examples are cited under degenerate rearrangement and fluxional.27 See [332].28 rev[3]29 revGB-revPOC3031 neighbouring-group participation32 Direct interaction of the reaction centre (usually, but not necessarily, an incipient 33 carbenium-ion centre) with a lone pair of electrons of an atom or with the electrons of a 34 or bond contained within the parent molecule but not conjugated with the reaction 35 centre.36 Example:

37

RS

X

RS

NuS

RS X Nu

38

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1 Note 1: A distinction is sometimes made between n, , and participation.2 Note 2: The neighbouring group serves as a nucleophile, as in SN2 reactions, except 3 that the nucleophile is intramolecular, so that this step is unimolecular.4 Note 3: A rate increase due to neighbouring-group participation is known as 5 anchimeric assistance.6 Note 4: Synartetic acceleration is the name given to the special case of participation 7 by electrons on a substituent attached to a -carbon, relative to the leaving group 8 attached to the -carbon, as in the example above. This term is deprecated.9 See also intramolecular catalysis, multi-centre bond.

10 rev[3]11 revGB1213 NHOMO14 Next-to-highest occupied molecular orbital.15 See subjacent orbital.16 rev[3]17 GB1819 NIH shift20 Intramolecular hydrogen migration that can be observed in enzymatic and chemical 21 hydroxylations of aromatic rings, as evidenced by appropriate deuterium labelling, as 22 in23

24

D

CH3

H

H

H

H

OH

CH3

D

H

H

H

hydroxylase

2526 Note 1: In enzymatic reactions the NIH shift is thought to derive from the 27 rearrangement of arene oxide intermediates, but other pathways have been suggested.28 Note 2: NIH stands for National Institutes of Health, where the shift was discovered.29 See [333].30 GB[3]3132 nitrene33 Generic name for the species HN and substitution derivatives thereof, containing an 34 electrically neutral univalent nitrogen atom with four nonbonding electrons.35 Note 1: Two nonbonding electrons may have antiparallel spins (singlet state) or 36 parallel spins (triplet state).

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1 Note 2: The name is the strict analogue to carbene and, as a generic name, it is 2 preferred to a number of alternative proposed (imene, imine radical, aminylene, azene, 3 azylene, azacarbene, imin, imidogen).4 See [97,334].5 GB[3]67 no-bond resonance8 double-bond-no-bond resonance9 Inclusion of one or more contributing structures that lack the bond of another

10 contributing structure.11 Example:

1213 See hyperconjugation.14 rev[3]15 GB1617 nonclassical carbocation18 Carbocation that has delocalized (bridged) bonding electrons.19 See [89,335].20 rev[3]21 revGB2223 non-Kekulé structure24 Compound with unpaired electrons for which no Lewis structures are possible with all 25 bonding electrons paired in single or double bonds.26 Examples:27

28 H2C CH2

2930 Note: For the second example ("trimethylenemethane") the isomer shown (valence 31 tautomer methylidenecyclopropane) is a Kekulé structure.32 See [336].33 diffGB

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12 nucleofuge3 Leaving group that carries away the bonding electron pair in a nucleophilic substitution 4 reaction.5 Example: In the hydrolysis of a chloroalkane, Cl– is the nucleofuge.67 R–Cl + H2O → ROH + HCl89 Note 1: Nucleofugality, commonly called leaving-group ability, characterizes the

10 relative rates of atoms or groups to depart with the bonding electron pair from a 11 reference substrate. Nucleofugality depends on the nature of the reference reaction and 12 is not the reverse of nucleophilicity.13 Note 2: Protofugality is a special type of nucleofugality, characterizing the relative 14 rates of proton transfer from a series of Brønsted acids H–X to a common Brønsted 15 base. 16 See [194].17 See also electrofuge, nucleophile.18 rev[3]19 revGB2021 nucleophile (n.), nucleophilic (adj.)22 Reactant that forms a bond to its reaction partner (the electrophile) by donating both of 23 its bonding electrons.24 Note 1: A "nucleophilic substitution reaction" is a heterolytic reaction in which the 25 reagent supplying the entering group acts as a nucleophile.26 Example:2728 MeO– (nucleophile) + EtCl → MeOEt + Cl– (nucleofuge)2930 Note 2: Nucleophilic reagents are Lewis bases.31 Note 3: The term "nucleophilic" is also used to designate the apparent polar character 32 of certain radicals, as inferred from their higher relative reactivity with reaction sites of 33 lower electron density.34 See also nucleophilicity, order of reaction.35 GB[3]3637 nucleophilic catalysis38 Catalysis by a Lewis base, involving conversion of a substrate with low electrophilicity 39 into an intermediate with higher electrophilicity. 40 Example 1: hydrolysis of acetic anhydride in aqueous solution, catalysed by 4-41 (dimethylamino)pyridine

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1 Me2NC5H4N + (CH3CO)2O → (Me2NC5H4NCOCH3)+ + CH3CO2–

2 then (Me2NC5H4COCH3)+ + H2O → Me2NC5H4N + CH3CO2H + H+aq

34 Example 2: SN2 reaction of CH3CH2Cl with HO–, catalyzed by I–:56 CH3CH2Cl + I– → Cl– + CH3CH2I, then CH3CH2I + HO– → CH3CH2OH + I–78 See also nucleophilicity.9 rev[3]

10 revGB-revPOC1112 nucleophilic substitution13 Heterolytic reaction in which an entering group adds to the electrophilic part of the 14 substrate and in which the leaving group, or nucleofuge, retains both electrons of the 15 bond that is broken, whereupon it becomes another potential nucleophile.16 Example:1718 CH3Br (substrate) + HO– (nucleophile) → CH3OH + Br– (nucleofuge)1920 Note 1: It is arbitrary to emphasize the nucleophile and ignore the feature that this is 21 also an electrophilic substitution, but the distinction depends on the nucleophilic nature 22 of the reactant that is considered to react with the substrate.23 Note 2: Nucleophilic substitution reactions are designated as SN1 or SN2, depending 24 on whether they are unimolecular or bimolecular, respectively. Mechanistically, these 25 correspond to two-step and one-step processes, respectively. SN1 reactions follow first-26 order kinetics but SN2 reactions do not always follow second-order kinetics.27 See order of reaction, rate coefficient.282930 nucleophilicity31 Relative reactivity of a nucleophile toward a common electrophile.32 Note 1: The concept is related to Lewis basicity. However, whereas the Lewis 33 basicity of a base B: is measured by its equilibrium constant for adduct formation with 34 a common acid A, the nucleophilicity of a Lewis base B: is measured by the rate 35 coefficient for reaction with a common substrate A–Z, often involving formation of a 36 bond to carbon.37 Note 2: Protophilicity is a special case of nucleophilicity, describing the relative rates 38 of reactions of a series of Lewis bases B: with a common Brønsted acid H–Z. The term 39 “protophilicity” is preferred over the alternative term “kinetic basicity” because basicity

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1 refers to equilibrium constants, whereas “philicity” (like “fugality”) refers to rate 2 constants.3 See [192,193,307].4 See also Brønsted basicity, electrophilicity, Lewis basicity, Mayr-Patz equation, 5 Ritchie equation, Swain-Scott equation.6 rev[3]7 revGB89 n-* delocalization (n-* no-bond resonance)

10 Delocalization of a lone pair (n) into an antibonding -orbital (*).11 See also anomeric effect, hyperconjugation, resonance.12 GB[3]1314 octanol-water partition ratio (KOW):15 Equilibrium concentration of a substance in octan-1-ol divided by its equilibrium 16 concentration in water. 17 Note: This is a measure of the lipophilicity of a substance. It is used in 18 pharmacological studies and in the assessment of environmental fate and transport of 19 organic chemicals.20 See [337,338].21 See also partition ratio.2223 octet rule24 Electron-counting rule that the number of lone-pair electrons on a first-row atom plus 25 the number of electron pairs in that atom's bonds should be 8.2627 onium ion28 (1) Cation derived by addition of a hydron to a mononuclear parent hydride of the 29 nitrogen, chalcogen, and halogen family, e.g., H4N+ ammonium ion.30 (2) Derivative formed by substitution of the above parent ions by univalent groups, e.g., 31 (CH3)2SH+ dimethylsulfonium (dimethylsulfanium), (CH3CH2)4N+ tetraethylammonium.32 See [97].33 See also carbenium ion, carbonium ion.34 rev[3]35 revGB3637 optical yield38 Ratio of the optical purity of the product to that of the chiral precursor or reactant.39 Note 1: This should not be confused with enantiomeric excess. 40 Note 2: The optical yield is not related to the chemical yield of the reaction.

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1 See [11]. 2 See stereoselectivity.3 GB[3]45 orbital (atomic or molecular)6 Wavefunction depending on the spatial coordinates of only one electron.7 Note: An orbital is often illustrated by sketching contours, often very approximate, on 8 which the wavefunction has a constant value or by indicating schematically the 9 envelope of the region of space in which there is an arbitrarily fixed high probability (say

10 95 %) of finding the electron occupying that region, and affixing also the algebraic sign 11 (+ or –) of the wavefunction in each part of that region, or suggesting the sign by 12 shading.13 Examples:14

151617 See atomic orbital, molecular orbital.18 See [8].19 rev[3]20 revGB-revPOC2122 orbital steering23 Concept expressing the principle that the energetically favourable stereochemistry of 24 approach of two reacting species is governed by the most favourable overlap of their 25 appropriate orbitals.26 rev[3]27 revGB-revPOC282930 orbital symmetry 31 Behaviour of an atomic orbital or molecular orbital under molecular symmetry 32 operations, such that under reflection in a symmetry plane or rotation by 180º around a 33 symmetry axis the phase of the orbital is either unchanged (symmetric) or changes sign 34 (antisymmetric), whereby positive and negative lobes are interchanged.35 Examples:36 (1) The orbital of an idealized single bond is , with cylindrical symmetry.

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1 (2) A p-orbital or -bond orbital has symmetry, i.e., it is antisymmetric with respect 2 to reflection in a plane passing through the atomic centres with which it is associated. 3 (3) The HOMO of 1,3-butadiene (illustrated below by its component atomic orbitals) 4 is antisymmetric with respect to 180º rotation about an axis through the C2–C3 bond 5 and perpendicular to the molecular plane.6789

101112 See [107,123].13 See also conservation of orbital symmetry, sigma, pi.14 rev[3]15 revGB-revPOC1617 order of reaction18 Exponent , independent of concentration and time, in the differential rate equation 19 (rate law) relating the macroscopic (observed, empirical, or phenomenological) rate of 20 reaction v to cA the concentration of one of the chemical species present, as defined by21

22 𝛼 = ( ∂ln {𝑣}

∂ln {cA})[B],…

23 The argument in the ln function should be of dimension 1. Thus, reduced quantities 24 should be used, i.e., the quantity divided by its unit, {v} = v/(mol dm-3 s-1) and = {𝑐A}25 cA/(mol dm-3).26 Note 1: A rate equation can often be expressed in the form v = k [A][B]... , 27 describing the dependence of the rate of reaction on the concentrations [A], [B],..., 28 where exponents , , ... are independent of concentration and time and k is 29 independent of [A], [B], ... . In this case the reaction is said to be of order with respect 30 to A, of order with respect to B,..., and of (total or overall) order n = + +... . The 31 exponents , ,... sometimes called "partial orders of reaction", can be positive or 32 negative, integral, or rational nonintegral numbers.33 Note 2: For an elementary reaction a partial order of reaction is the same as the 34 stoichiometric number. The overall order is then the same as the molecularity. For 35 stepwise reactions there is no general connection between stoichiometric numbers and 36 partial orders. Such reactions may have more complex rate laws, so that an apparent 37 order of reaction may vary with the concentrations of the chemical species involved and

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1 with the progress of the reaction: in such cases it is not useful to speak of orders of 2 reaction, although apparent orders of reaction may be deducible from initial rates.3 Note 3: In a stepwise reaction, orders of reaction may in principle be assigned to the 4 elementary steps.5 Note 4: For chemical rate processes occurring in systems for which concentration

6 changes are not measurable, as in the case of a dynamic equilibrium aA ⇄ pP, and if a

7 chemical flux –A is found experimentally (e.g., by NMR line-shape analysis) to be 8 related to the concentration of A and to concentrations of other species B, ..., by the 9 equation

1011 –A = k[A][B] ...1213 then the reaction is of order α with respect to A... and of total (or overall) order 14 (= + +...). 15 Note 5: If the overall rate of reaction is given by1617 v = k[A][B]1819 but [B] remains constant in any particular sample (but can vary from sample to sample), 20 then the order of the reaction in A will be, and the rate of disappearance of A can be 21 expressed in the form2223 vA = kobs[A]2425 The proportionality factor kobs is called the "observed rate coefficient" and is related to 26 the rate constant k by2728 kobs = k[B]

2930 Note 6: For the frequent case where = 1, kobs is often referred to as a "pseudo-first-31 order rate coefficient".32 See [13].33 See also kinetic equivalence, rate coefficient, rate constant.34 rev[3]35 revGB-revPOC36373839 organocatalysis

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1 Catalysis by small organic molecules, as distinguished from catalysis by (transition) 2 metals or enzymes. 3 Note 1: Frequently used organocatalysts are secondary amines (covalent catalysis 4 by generation of enamines or of iminium ions as reactive intermediates) and thioureas 5 (hydrogen-bonding catalysis). 6 Note 2: Organocatalysts are often employed for enantioselectivity.7 Note 3: The mechanisms employed by organocatalysts are examples of general acid 8 catalysis, general base catalysis, nucleophilic catalysis, specific acid catalysis, specific 9 base catalysis.

10 See [339,340,341,342].1112 outer-sphere electron transfer13 Feature of an electron transfer between redox centres not sharing a common atom or 14 group.15 Example:16 MnO4

– + Mn*O42– ⇄ MnO4

2– + Mn*O4–

17 Note 1: In the transition state the interaction between the relevant electronic orbitals 18 of the two centres is weak (below 20 kJ mol–1), and the electron(s) must tunnel through 19 space.20 Note 2: If instead the donor and the acceptor exhibit a strong electronic coupling, 21 often through a ligand that bridges both, the reaction is described as inner-sphere 22 electron transfer. 23 Note 3: These two terms derive from studies of metal complexes, and for organic 24 reactions the terms "nonbonded' and 'bonded" electron transfer are often used.25 See [9,182,343].26 rev[3]27 GB2829 oxidation30 (1) Removal of one or more electrons from a molecular entity. 31 (2) Increase in the oxidation number of an atom within a substrate, see [344].32 (3) Gain of oxygen and/or loss of hydrogen by an organic substrate.33 Note 1: All oxidations meet criterion (2) and many meet criterion (3), but this is not 34 always easy to demonstrate. Alternatively, an oxidation can be described as the 35 transformation of an organic substrate by removal of one or more electrons from the 36 substrate, often accompanied by gain or loss of water, hydrons, and/or hydroxide, or by 37 nucleophilic substitution, or by molecular rearrangement.38 Note 2: This formal definition allows the original idea of oxidation (combination with 39 oxygen), together with its extension to removal of hydrogen, as well as processes 40 closely akin to this type of transformation (and generally regarded in organic chemistry

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1 to be oxidations and to be effected by "oxidizing agents") to be descriptively related to 2 definition (1). For example the oxidation of methane to chloromethane may be 3 considered as4 CH4 – 2e– – H+ + HO– = CH3OH (oxidation), 5 followed by 6 CH3OH + HCl = CH3Cl + H2O (neither oxidation nor reduction)7 rev[3]8 revGB-revPOC9

10 oxidation number11 oxidation state12 Number assigned to a carbon atom in a covalent organic compound according to1314 NOx = NX + NO + NN – NH – NM

1516 where NX, NO, NN, NH, and NM are the numbers of bonds to halogen, oxygen (or sulfur), 17 nitrogen, hydrogen, and a metal, respectively.18 Examples:1920 CH3MgBr (-4), CH3CH3 (-3), CH2=CH2 (-2), CH3OH (-2), CH2Cl2 (0),21 CH3CH=O (+1), HCN (+2), CCl4 (+4). 2223 Note 1: This assignment is based on the convention that each attached atom more 24 electronegative than carbon contributes +1, while each atom less electronegative 25 (including H) contributes -1, and an attached carbon contributes zero.26 Note 2: Oxidation numbers are not significant in themselves, but changes in oxidation 27 number are useful for recognizing whether a reaction is an oxidation or a reduction or 28 neither.29 Note 3: A different system is used for transition-metal species [345].30 See also electronegativity, oxidation.31 rev[3]32 diffGB3334 oxidative addition35 Insertion of the metal of a metal complex into a covalent bond involving formally an 36 overall two-electron loss on one metal or a one-electron loss on each of two metals, 37 i.e.,

38 LnMm + XY → LnMm+2(X)(Y)

39 or 2 LnMm + XY → LnMm+1(X) + LnMm+1(Y)

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1 rev[3]2 revGB-revPOC34 oxidative coupling5 Coupling of two molecular entities through an oxidative process, usually catalysed by a 6 transition-metal compound.7 Example (where the oxidant can be O2 or Cu(II) or others):8

910 rev[3]11 revGB-revPOC1213 parallel effect14 Change of the position of the transition state upon stabilization or destabilization of a 15 structure (or structures) along the assumed minimum-energy reaction path.16 See Hammond postulate, More O'Ferrall - Jencks diagram.1718 parallel reaction19 See composite reaction.20 GB[3]2122 paramagnetism23 Property of substances having a magnetic susceptibility greater than 0, whereby they 24 are drawn into a magnetic field.25 See also diamagnetism.26 GB[3]2728 partial rate factor pf

Z, mfZ

29 Rate constant for substitution at one specific site in an aromatic compound divided by 30 the rate constant for substitution at one position in benzene.31 Note 1: The partial rate factor pf

Z for para-substitution in a monosubstituted benzene 32 C6H5Z is related to the rate constants k(C6H5Z) and k(C6H6) for the total reactions (i.e., 33 at all positions) of C6H5Z and benzene, respectively, and fpara (the fraction of para-34 substitution in the total product formed from C6H5Z, usually expressed as a percentage) 35 by the relation36

37 𝑝Z𝑓 =

6𝑘(C6H5Z)𝑘(C6H6) 𝑓𝑝𝑎𝑟𝑎

38

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1 Similarly for meta-substitution:2

3 𝑚Z𝑓 =

6𝑘(C6H5Z)2𝑘(C6H6) 𝑓𝑚𝑒𝑡𝑎

45 The symbols fp

Z, fmZ, fo

Z are also in use.6 Note 2: The term applies also to the ipso position, and it can be extended to other 7 substituted substrates undergoing parallel reactions at different sites with the same 8 reagent according to the same rate law.9 See [262,346,347].

10 See also selectivity.11 rev[3]12 revGB-revPOC1314 partition ratio (partition constant, distribution ratio) P 15 Concentration of a substance in one phase divided by its concentration in another 16 phase, at equilibrium.17 Example, for an aqueous/organic system the partition ratio (or distribution ratio D) is 18 given by19 P = corg(A)/caq(A)20 Note 1: The most common way of applying P in correlation analysis or quantitative 21 structure-activity relationships is as lg P.22 Note 2: The parameter P is extensively used as an indicator of the capacity of a 23 molecular entity to cross biological membranes by passive diffusion.24 Note 3: The term partition coefficient is in common usage in toxicology but is not 25 recommended for use in chemistry and should not be used as a synonym for partition 26 constant, partition ratio, or distribution ratio. 27 See [167,232].28 See also Hansch constant, octanol-water partition ratio.29 diffGB3031 pericyclic reaction32 Chemical reaction in which concerted reorganization of bonding takes place throughout 33 a cyclic array of continuously bonded atoms.34 Note 1: It may be viewed as a reaction proceeding through a fully conjugated cyclic 35 transition state.36 Note 2: The term embraces a variety of processes, including cycloaddition, 37 cheletropic reaction, electrocyclic reaction and sigmatropic rearrangement, etc. 38 (provided they are concerted).39 See also pseudopericyclic.

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1 GB[3]23 permittivity, relative εr

4 dielectric constant (obsolete)5 Measure of the reduction of the magnitude of the potential energy of interaction between 6 two charges on going from vacuum to a condensed medium, expressed as the ratio of 7 the former to the latter.8 Note: The term dielectric constant is obsolete. Moreover, the dielectric constant is 9 not a constant since it depends on frequency.

10 See [12].11 GB1213 perpendicular effect14 Change of the position of the transition state upon stabilization or destabilization of a 15 structure (or structures) that lies off the assumed minimum-energy reaction path.16 See anti-Hammond effect, Hammond postulate, More O'Ferrall - Jencks diagram.17 rev[3]18 revGB-revPOC1920 persistence21 Characteristic of a molecular entity that has an appreciable lifetime (minutes or 22 nanoseconds or other, depending on context).23 Note 1: Dilute solution or inert solvent may be required for persistence.24 Note 2: Persistence is a kinetic or reactivity property, whereas, in contrast, stability 25 (being stable) is a thermodynamic property.26 See [348].27 See also transient.28 rev[3]29 revGB-revPOC3031 pH-rate profile32 Plot of observed rate coefficient, or more usually the decadic logarithm of its numerical 33 value, against solution pH, other variables being kept constant.34 GB[3]3536 phase-transfer catalysis37 Enhancement of the rate of a reaction between chemical species located in different 38 phases (immiscible liquids or solid and liquid) by addition of a small quantity of an agent 39 (called the phase-transfer catalyst) that extracts one of the reactants, most commonly 40 an anion, into the other phase so that reaction can proceed.

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1 Note 1: These catalysts are often onium ions (e.g., tetraalkylammonium ions) or 2 complexes of inorganic cations (e.g., as crown ether complexes).3 Note 2: The catalyst cation is not consumed in the reaction although an anion 4 exchange does occur.5 Example:6 CH3(CH2)7Cl in decane + aqueous Na+CN– + catalytic PhCH2N(CH2CH3)3

+ 7 = CH3(CH2)7CN + aqueous Na+Cl–8 rev[3]9 revGB-revPOC

1011 phenonium ion12 See bridged carbocation.13 GB[3]1415 photochromism16 Reversible transformation of a molecular entity between two forms, A and B, having 17 different absorption spectra, induced in one or both directions by absorption of 18 electromagnetic radiation.19 Note 1: The thermodynamically stable form A is transformed by irradiation into form 20 B. The back reaction can occur thermally (photochromism of type T) or photochemically 21 (photochromism of type P).22 Note 2: The spectral change is typically, but not necessarily, of visible colour.23 Note 3: An important parameter is the number of cycles that a photochromic system 24 can undergo.25 See [9].2627 GB2829 photolysis30 Bond cleavage induced by ultraviolet, visible, or infrared radiation. 31 Example:32 Cl2 → 2 Cl33 Note: The term is used incorrectly to describe irradiation of a sample without any 34 bond cleavage, although in the term “flash photolysis” this usage is accepted.35 See [9].36 GB[3]3738 photostationary state

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1 Steady state reached by a chemical system undergoing photochemical reaction, such 2 that the rates of formation and disappearance are equal for each of the transient 3 molecular entities formed.4 See [9].5 GB67 pi-adduct8 See -adduct.9

10 pi-bond11 See , .121314 polar aprotic solvent15 See dipolar non-HBD solvent.16 GB[3]1718 polar effect19 All the interactions whereby a substituent on a reactant molecule RY modifies the 20 electrostatic forces operating at the reaction centre Y, relative to the reference standard 21 RoY.22 Note 1: These forces may be governed by charge separations arising from 23 differences in the electronegativity of atoms (leading to the presence of dipoles), by the 24 presence of monopoles, or by electron delocalization.25 Note 2: It is distinguished from a steric effect.26 Note 3: Sometimes, however, the term polar effect is taken to refer to the influence, 27 other than steric, that non-conjugated substituents exert on reaction rates or equilibria, 28 thus excluding effects of electron delocalization between a substituent and the 29 molecular framework to which it is attached. 30 See also electronic effects (of substituents), field effect, inductive effect.31 GB[3]3233 polar solvent34 Liquid composed of molecules with a significant dipole moment, capable of dissolving 35 ions or other molecules with significant dipole moments.36 See polarity.37 rev[3]38 revGB3940 polarity (of a bond)

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1 Characteristic of a bond between atoms of different electronegativity, such that the 2 electrons in that bond are not shared equally.34 polarity (of a solvent)5 Overall solvation capability (solvation power) of a solvent toward solutes, which 6 depends on the action of all possible intermolecular interactions between solute ions or 7 molecules and solvent molecules, excluding interactions leading to definite chemical 8 alterations of the ions or molecules of the solute.9 Note: Quantitative measures of solvent polarity include relative permittivity (dielectric

10 constant) and various spectroscopic parameters.11 See [17].12 See solvent parameter.13 rev[3]14 revGB-revPOC1516 polarizability17 18 (SI unit: C m2 V–1)19 electric polarizability 20 Induced dipole moment, induced, divided by applied electric field strength E21 = induced/E 22 Note 1: The polarizability represents the ease of distortion of the electron cloud of a 23 molecular entity by an electric field (such as that due to the proximity of a charged 24 species).25 Note 2: Polarizability is more often expressed as polarizability volume, with unit cm3, 26 where 0 is the permittivity of vacuum:27

28 /cm3 = 106

4π𝜀0

d𝝁d𝐸

2930 Note 3: In general, polarizability is a tensor that depends on direction, for example, 31 depending on whether the electric field is along a bond or perpendicular to it, and the 32 induced dipole may not even be along the direction of the electric field. However, in 33 ordinary usage the term refers to the mean polarizability, the average over three 34 rectilinear axes of the molecule.35 rev[3]36 revGB-revPOC3738 polydent39 polydentate

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1 See ambident.2 GB[3]34 potential-energy profile5 Curve describing the variation of the potential energy of a system of atoms as a function 6 of a single coordinate.7 Note 1: For an elementary reaction the relevant coordinate is the reaction coordinate, 8 which is a measure of progress along the minimum-energy reaction path (MERP) from 9 a saddle point on a potential-energy surface in each direction toward adjacent energy

10 minima. For a stepwise reaction it is the succession of reaction coordinates for the 11 successive individual reaction steps. For a reaction involving a bifurcation each branch 12 requires a different reaction coordinate and has its own profile.13 Note 2: A profile constructed as a function of an arbitrary internal coordinate (for 14 example, a bond distance) is not guaranteed to pass through a saddle point on the 15 corresponding potential-energy surface; in order to do so, it must be smooth, 16 continuous, and follow the path of lowest energy connecting the reactant and product 17 energy minima.18 Examples: (one-step reaction, two-step reaction)19

202122 See [349].23 See also Gibbs energy diagram, potential-energy surface.24 rev[3]25 revGB-rev POC2627 potential-energy surface (PES)28 Surface describing the variation within the Born-Oppenheimer approximation of the 29 potential energy of a system of atoms as a function of a set of internal coordinates. 30 Note 1: A minimum on a PES is characterized by positive curvature in all directions 31 and corresponds to a structure that is stable with respect to small displacements away 32 from its equilibrium geometry; for this structure (a reactant, intermediate or product) all 33 vibrational frequencies are real. A saddle point is characterized by positive curvature in

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1 all directions except for one with negative curvature and corresponds to a transition 2 structure, for which one vibrational frequency is imaginary. A local maximum is 3 characterized by negative curvatures in two (or more) directions and has two (or more) 4 imaginary frequencies; it is sometimes called a second-order (or higher-order) saddle 5 point.6 Note 2: It is usual to select only two coordinates in order to represent the surface, 7 with potential energy as the third dimension, or alternatively as a two-dimensional 8 contour map. For example, a PES for a simple reacting triatomic system A–B + C → 9 A + B–C, could be constructed using the A···B and B···C distances as two internal

10 coordinates; the third independent coordinate could be the ABC angle or the A···C 11 distance, and its value could either be kept fixed or else be relaxed to minimize the 12 energy at each point on the (A···B, B···C) surface.13 Note 3: The path of steepest-descent from a saddle point in each direction towards 14 adjacent energy minima defines a minimum-energy reaction path (MERP) that is 15 equivalent to the energetically easiest route from reactants to products. The change in 16 potential energy along this path across the PES defines a potential-energy profile for 17 the elementary reaction. Progress along this path is measured by the value of the 18 reaction coordinate.19 Note 4: In general there is neither a unique set of internal coordinates nor a unique 20 choice of two coordinates with which to construct a PES of reduced dimensionality. 21 Consequently there is no guarantee of a smooth and continuous PES containing a 22 saddle point connecting reactant and product minima. 23 See [8,73,350,351].24 See: bifurcation, minimum-energy reaction path, potential-energy profile, reaction 25 coordinate, transition structure.26 rev[3]27 revGB-revPOC2829 potential of mean force (PMF)30 Free energy as a function of a set of coordinates, the negative gradient of which gives 31 the average force acting on that configuration averaged over all other coordinates and 32 momenta within a statistical distribution. If the averaging is performed within a canonical 33 ensemble (constant volume, temperature, and number of particles) the PMF is 34 equivalent to the Helmholtz energy, but if it is performed within an isobaric-isothermal 35 ensemble (constant pressure, temperature, and number of particles) the PMF is 36 equivalent to the Gibbs energy. 37 Note 1: Commonly, the PMF acting upon a selected geometric variable and averaged 38 over the coordinates and momenta of all other geometric variables is evaluated for a 39 succession of constrained values of the selected variable, thereby generating 40 (generically) a free-energy profile with respect to the selected reaction coordinate (e.g.,

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1 a bond distance or angle, or a combination of internal coordinates); specifically this is 2 either a Helmholtz-energy or a Gibbs-energy profile, depending upon the choice of 3 ensemble for the statistical averaging within a computational simulation. 4 Note 2: Selection of two geometric variables as reaction coordinates allows a free-5 energy surface to be computed as a two-dimensional PMF.6 Note 3: Molecular simulations often yield Helmholtz energies, not Gibbs energies, 7 but for condensed phases the difference is usually neglected.8 See: free energy, reaction coordinate.9

10 pre-association11 Step on the reaction path of some stepwise reactions in which the molecular entity C 12 forms an encounter pair or encounter complex with A prior to the reaction of A to form 13 product.14 Example:

151617 Note 1: In this mechanism the chemical species C may but does not necessarily 18 assist the formation of B from A, which may itself be a bimolecular reaction with some 19 other reagent.20 Specific example (aminolysis of phenyl acetate):21

222324 Note 2: Pre-association is important when B is too short-lived to permit B and C to 25 come together by diffusion. In the specific example, T± would dissociate faster than 26 general acid HA can diffuse to it. Experimentally, the Brønsted α is > 0, which is 27 inconsistent with rate-limiting diffusion and hydron transfer.

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1 See [352].2 See also Brønsted relation, microscopic diffusion control, spectator mechanism.3 rev[3]4 revGB-revPOC56 precursor complex7 See encounter complex.8 revGB-revPOC[3]9

10 pre-equilibrium11 prior equilibrium12 Rapid reversible step preceding the rate-limiting step in a stepwise reaction.13 Example:

141516 See also kinetic equivalence, steady state.17 GB[3]1819 pre-exponential factor20 See energy of activation, entropy of activation.21 GB[3]2223 principle of nonperfect synchronization24 Consideration applicable to reactions in which there is a lack of synchronization 25 between bond formation or bond rupture and other changes that affect the stability of 26 products and reactants, such as resonance, solvation, electrostatic, hydrogen bonding 27 and polarizability effects. 28 Note: The principle states that a product-stabilizing factor whose development lags 29 behind bond changes at the transition state, or a reactant-stabilizing factor whose loss 30 is ahead of bond changes at the transition state, increases the intrinsic barrier and 31 decreases the rate constant of a reaction. For a product-stabilizing factor whose 32 development is ahead of bond changes, or a reactant-stabilizing factor whose loss lags 33 behind bond changes, the opposite relations hold. The reverse effects are observable 34 for factors that destabilize a reactant or product. 35 See [108].36 See also imbalance, synchronous.37 revGB38

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1 prior equilibrium2 See pre-equilibrium.3 GB[3]45 product-determining step6 Step of a stepwise reaction in which the product distribution is determined.7 Note: The product-determining step may be identical to, or may occur later than, the 8 rate-determining step in the reaction.9 GB[3]

1011 product-development control12 Case of kinetic control in which the selectivity of a reaction parallels the relative 13 (thermodynamic) stabilities of the products.14 Note: Product-development control arises because whatever effect stabilizes or 15 destabilizes a product is already operative at the transition state. Therefore it is usually 16 associated with a transition state occurring late on the minimum-energy reaction path.17 See also steric-approach control, thermodynamic control.18 GB[3]1920 promotion21 See pseudo-catalysis.22 GB[3]2324 propagation25 See chain reaction.26 GB[3]2728 propargylic substitution29 See allylic substitution reaction.3031 protic32 See protogenic.33 GB[3]3435 protic solvent36 Solvent that is capable of acting as a hydrogen-bond donor.37 See HBD solvent.3839 protofugality

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1 special case of nucleofugality, describing the relative rates of transfer of a proton (more 2 generally: hydron) from a series of Brønsted acids H–X to a common Brønsted base. 3 Note: This term has the advantage over the commonly used “kinetic acidity” that 4 philicity and fugality are associated with kinetics, while acidity and basicity are 5 associated with thermodynamics.6 See [194].7 See also Brønsted acidity, nucleofugality, protophilicity. 89 protogenic (solvent)

10 HBD (hydrogen bond donor) solvent.11 Capable of acting as a proton (hydron) donor.12 Note 1: Such a solvent may be a strong or weak Brønsted acid.13 Note 2: The term is preferred to the synonym protic or to the more ambiguous 14 expression acidic.15 See protophilic solvent.16 [3]1718 protolysis19 proton (hydron)-transfer reaction.20 Note: Because of its misleading similarity to hydrolysis, photolysis, etc., this use is 21 discouraged.22 See also autoprotolysis.23 rev[3]24 GB2526 proton affinity27 Negative of the enthalpy change in the gas phase reaction between a proton (more 28 appropriately hydron) and the chemical species concerned, usually an electrically 29 neutral or anionic species, to give the conjugate acid of that species.30 Note 1: For an anion A–, the proton affinity is the negative of the enthalpy of the 31 heterolytic dissociation (in the gas phase) of the Brønsted acid HA.32 Note 2: Proton affinity is often, but unofficially, abbreviated as PA.33 Note 3: Affinity properly refers to Gibbs energy.34 See also gas phase basicity, gas phase acidity.35 See [2]. See also [213,353].36 rev[3]37 revGB-revPOC3839 protonation40 Attachment of the ion 1H+ (of relative atomic mass 1).

Commented [s1]: Alphabetic reorganization

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1 See also hydronation.23 proton–transfer reaction4 Chemical reaction, the main feature of which is the intermolecular or intramolecular 5 transfer of a proton (hydron) from one binding site to another.6 Example:7 CH3COOH + (CH3)2C=O → CH3CO2

– + (CH3)2C=OH+

8 Note: In the detailed description of proton-transfer reactions, especially of rapid 9 proton transfers between electronegative atoms, it should always be specified whether

10 the term is used to refer to the overall process (including the more-or-less encounter-11 controlled formation of a hydrogen-bonded complex and the separation of the products) 12 or just to the proton-transfer event (including solvent rearrangement) by itself.13 See also autoprotolysis, microscopic diffusion control, tautomerism.14 GB[3]1516 protophilic (solvent)17 See HBA (hydrogen bond acceptor) solvent.18 rev[3]19 revGB20212223 protophilicity24 Special case of nucleophilicity, describing the relative rates of reactions of a series of 25 Lewis bases with a common Brønsted acid. This term has the advantage over the 26 commonly used “kinetic basicity” that philicity and fugality are associated with kinetics, 27 while acidity and basicity are associated with thermodynamics.28 See [194].29 See also Brønsted basicity, nucleophilicity, protofugality.3031 prototropic rearrangement (prototropy)32 See tautomerization.33 GB[3]3435 pseudo-catalysis36 Increase of the rate of a reaction by an acid or base present in nearly constant 37 concentration throughout a reaction in solution (owing to buffering or to the use of a 38 large excess), even though that acid or base is consumed during the process, so that 39 the acid or base is not a catalyst and the phenomenon strictly cannot be called catalysis 40 according to the established meaning of these terms in chemical kinetics.

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1 Note 1: Although the mechanism of such a process is often closely related to that of 2 a catalysed reaction, it is recommended that the term pseudo-catalysis be used in these 3 and analogous cases. For example, if a Brønsted acid accelerates the hydrolysis of an 4 ester to a carboxylic acid and an alcohol, this is properly called acid catalysis, whereas 5 the acceleration, by the same acid, of the hydrolysis of an amide should be described 6 as pseudo-catalysis because the acid pseudo-catalyst is stoichiometrically consumed 7 during the reaction through formation of an ammonium ion.8 Note 2: The terms general-acid pseudo-catalysis and general-base pseudo-catalysis 9 may be used as the analogues of general acid catalysis and general base catalysis.

10 Note 3: The terms acid- and base-promoted, acid- and base-accelerated, and acid- 11 and base-induced are sometimes used for reactions that are pseudo-catalysed by or 12 bases. However, the term promotion also has a different meaning in other chemical 13 contexts.14 rev[3]15 GB1617 pseudo-first-order rate coefficient18 See order of reaction, rate coefficient.19 GB[3]2021222324 pseudopericyclic25 Feature of a concerted transformation in which the primary changes in bonding occur 26 within a cyclic array of atoms but in which one (or more) nonbonding and bonding 27 atomic orbitals interchange roles.28 Examples: enol-to-enol tautomerism of 4-hydroxypent-3-en-2-one (where the 29 electron pairs in the O–H bond and in the lone pair on the other O are in σ orbitals 30 whereas the other three electron pairs are in orbitals) and hydroboration (where the 31 B uses an sp2 bonding orbital and a vacant p orbital)32

333435 Note: Because the atomic orbitals that interchange roles are orthogonal, such a 36 reaction does not proceed through a fully conjugated transition state and is thus not a 37 pericyclic reaction. It is therefore not governed by the rules that express orbital 38 symmetry restrictions applicable to pericyclic reactions.

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1 See [354,355].2 See also coarctate.3 rev[3]4 revGB-revPOC56 push-pull conjugation7 Feature of an extended conjugated system bearing an electron donor group at one 8 end and an electron acceptor group at the other end.9 See [356].

10 See also cross-conjugation.1112 pyrolysis13 Thermolysis, usually associated with exposure to a high temperature.14 See also flash vacuum pyrolysis.15 GB[3]1617 -adduct (pi-adduct)18 Adduct formed by electron-pair donation from a orbital into a * orbital, or from a 19 orbital into a * orbital, or from a orbital into a * orbital. 20 Example:

2122 Note: Such an adduct has commonly been known as a complex, but, as the 23 bonding is not necessarily weak, it is better to avoid the term complex, in accordance 24 with the recommendations in this Glossary.25 See also coordination.26 GB[3]2728 -bond (pi bond)29 Interaction between two atoms whose p orbitals overlap sideways.30 Note: The designation as is because the p orbitals are antisymmetric with respect 31 to a defining plane containing the two atoms. 32 See sigma, pi.33 rev[3]34 revGB-rev POC

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12 -complex3 See -adduct.4 GB[3]56 -electron acceptor7 Substituent capable of electron withdrawal by resonance (e.g., NO2).8 See electronic effect, polar effect, -electron donor, -constant.9 rev[3]

10 revGB-revPOC1112 -electron donor13 Substituent capable of electron donation by resonance (e.g., OCH3).14 See electronic effect, polar effect, -electron acceptor, -constant.15 rev[3]16 revGB1718 quantitative structure-activity relationship (QSAR)19 Regression model to correlate biological activity or chemical reactivity with predictor 20 parameters based on measured or calculated features of molecular structure.21 See [357,358].22 See also correlation analysis.23 rev[3]24 revGB-revPOC2526 quantitative structure-property relationship (QSPR)27 Regression model to correlate chemical properties such as boiling point or 28 chromatographic retention time with predictor parameters based on measured or 29 calculated features of molecular structure.30 See [359].31 See also correlation analysis.3233 quantum yield34 Number of defined events that occur per photon absorbed by the system.35 Note 1: The integral quantum yield is the number of events divided by the number 36 of photons absorbed in a specified wavelength range. 37 Note 2: For a photochemical reaction () is the amount of reactant consumed or 38 product formed divided by the number of photons absorbed at wavelength.39 Note 3: The differential quantum yield for a homogeneous system is40

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1 𝛷(𝜆) = |d𝑥d𝑡|

𝑞𝑝,𝜆[1 ― 10𝐴(𝜆)]23 where |dx/dt| is the rate of change of a quantity x that measures the progress of a

4 reaction, qp, is the spectral photon flux (mol or its non-SI equivalent einstein) incident 5 per unit time at wavelength , and A() is the decadic absorbance at the excitation 6 wavelength.7 Note 4: When the quantity x is an amount concentration, it is convenient to use in 8 the denominator the rate (in moles) of photons absorbed per volume. 9 See [9,10].

10 rev[3]11 revGB-revPOC1213 radical14 free radical (obsolete)15 Molecular entity possessing an unpaired electron.16 Examples: •CH3, •SnR3, Cl•17 Note 1: In these formulae the dot, symbolizing the unpaired electron, should be 18 placed so as to indicate the atom of highest spin density, if possible.19 Note 2: Paramagnetic metal ions are not normally regarded as radicals. However, in 20 the isolobal analogy the similarity between certain paramagnetic metal ions and radicals 21 becomes apparent.22 Note 3: Depending upon the core atom that possesses the highest spin density, the 23 radicals can be described as carbon-, oxygen-, nitrogen-, or metal-centred radicals.24 Note 4: If the unpaired electron occupies an orbital having considerable s or more or 25 less pure p character, the respective radicals are termed or radicals.26 Note 5: The term radical has also been used to designate a substituent group within 27 a molecular entity, as opposed to "free radical", which is now simply called radical. The 28 bound entities may be called groups or substituents, but should no longer be called 29 radicals.30 See [40,97].31 See also diradical.32 rev[3]33 GB3435 radical combination36 Formation of a covalent bond by reaction of one radical with another.37 See colligation.

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1 rev[3]2 revGB34 radical ion5 Radical that carries a net electric charge.6 Note 1: A positively charged radical is called a radical cation (e.g., the benzene 7 radical cation C6H6

•+); a negatively charged radical is called a radical anion (e.g., the 8 benzene radical anion C6H6

•– or the benzophenone radical anion Ph2C–O•–).9 Note 2: Unless the positions of unpaired spin and charge can be associated with

10 specific atoms, superscript dot and charge designations should be placed in the order 11 •+ or •–, as suggested by the name radical ion. However, the usage in mass 12 spectrometry is to place the charge symbol before the dot [7].13 Note 3: In the first edition of this Glossary it was recommended to place the charge 14 designation directly above the dot. This format is now discouraged because of the 15 difficulty of extending it to ions bearing more than one charge and/or more than one 16 unpaired electron.17 GB[3]1819 radical pair20 geminate pair21 Two radicals in close proximity in solution, within a solvent cage.22 Note 1: The two radicals may be formed simultaneously by some unimolecular 23 process, e.g., peroxide decomposition or photolysis, or they may have come together 24 by diffusion.25 Note 2: While the radicals are together, correlation of the unpaired electron spins of 26 the two species cannot be ignored: this correlation is responsible for the CIDNP 27 phenomenon.28 See also geminate recombination.29 See [9,40].30 GB[3]3132 radiolysis33 Cleavage of one or several bonds resulting from exposure to high-energy radiation.34 Note: The term is also often used loosely to specify the method of irradiation (e.g., 35 pulse radiolysis) used in any radiochemical reaction, not necessarily one involving bond 36 cleavage.37 GB[3]3839 rate coefficient

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1 Empirical constant k in the equation for the rate of a reaction that is expressible by an 2 equation of the form34 v = k[A]a[B]b....56 Note 1: It is recommended that the term rate constant be confined to reactions that 7 are believed to be elementary reactions.8 Note 2: When a rate coefficient relates to a reaction occurring by a composite 9 mechanism, it may vary not only with temperature and pressure but also with the

10 concentration of reactants. For example, in the case of a unimolecular gas reaction the 11 rate at sufficiently high pressures is given by1213 v = k[A]1415 whereas at low pressures the rate expression is1617 v = k'[A]21819 Similarly, for a second-order reaction, the rate is given by2021 v = k2[A][B]2223 But under conditions where [B] remains constant at [B]0, as when B is a catalyst or is 24 present in large excess, 2526 v = k2[A][B]0 = k[A]2728 where the rate coefficient k = k2[B]0, which varies with [B]0.29 Such a rate coefficient k, which varies with the concentration [B], is called a first-30 order rate coefficient for the reaction, or a pseudo first-order rate constant even though 31 it is not a rate constant.32 Note: Rate constant and rate coefficient are often used as synonyms, see [12], 33 section 2.12, p.63.34 See [13].35 See order of reaction.36 rev[3]37 revGB-revPOC3839 rate constant40 k

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1 Term generally used for the rate coefficient of a reaction that is believed to be an 2 elementary reaction. See [13].3 Note 1: In contrast to a rate coefficient, a rate constant should be independent of 4 concentrations, but in general both rate constant and rate coefficient vary with 5 temperature.6 Note 2: Rate constant and rate coefficient are often used as synonymous, see [12], 7 section 2.12, p.63.8 See order of reaction.9 rev[3]

10 revGB-revPOC1112 rate-controlling step 13 See rate-determining states, rate-limiting step.14 rev[3]15 GB1617 rate law18 empirical differential rate equation19 Expression for the rate of reaction in terms of concentrations of chemical species and 20 constant parameters (normally rate coefficients and partial orders of reaction) only.21 For examples of rate laws see the equations under kinetic equivalence, and under 22 steady state.23 GB[3]2425 rate-limiting step26 rate-controlling step27 rate-determining step28 Step in a multistep reaction that is the last step in the sequence whose rate constant 29 appears in the rate equation.30 See [13].31 Example: Two-step reaction of A with B to give intermediate C, which then reacts 32 further with D to give products:

3334 If [C] reaches a steady state, then the observed rate is given by3536 v = –d[A]/dt = k1k2[A][B][D]/(k–1 + k2[D])37

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1 Case 1: If k2[D] >> k–1,then the observed rate simplifies to23 v = –d[A]/dt = k1[A][B]45 Because k2 disappears from the rate equation and k1 is the last rate constant to remain, 6 step (1) is said to be rate-limiting.7 Case 2: If k2[D] << k–1, then the observed rate is given by89 v = k1k2[A][D]/k–1 = Kk2[A][B][D]

1011 where K, equal to k1/k–1, is the equilibrium constant for the pre-equilibrium (Step 1). 12 Because k2 remains in the rate equation, Step 2 is said to be rate-limiting. Notice that 13 in this case, where Step 2 involves another reactant D, which step is rate-limiting can 14 depend on [D]: Step 1 at high [D] and Step 2 at low [D].1516 Specific example: Acid-catalyzed bromination of a methyl ketone17

181920 where k1 = k1'[H+], k–2 = k–2'[H+], k3 = k3'[Br2], k–3 = k–3'[Br–].2122 According to the steady-state approximation,2324 v = kobs[RCOCH3] = k1k2k3k4[RCOCH3]/{k2k3k4 + k–1k3k4 + k–1k–2(k–3 + k4)}.2526 or kobs = k1k2k3k4/{k2k3k4 + k–1k3k4 + k–1k–2(k–3 + k4)}.2728 If k4 >> k–3, k–3 can be ignored in the parentheses, so that kobs simplifies to 29 k1k2k3k4/{k2k3k4 + k–1k3k4 + k–1k–2k4}. Then if k3 >> k–2, kobs simplifies further to 30 k1k2k3k4/{k2k3k4 + k–1k3k4}, where k3k4 cancels and kobs becomes k1k2/{k–1 + k2}, so that 31 k2 is the last rate constant remaining in kobs. The second step is rate-limiting, and the 32 rate is independent of k3 or of [Br2].33 Note 1: Although the expressions rate-controlling, rate-determining, and rate-limiting 34 are often regarded as synonymous, rate-limiting is to be preferred, because in Case 2 35 all three rate constants enter into the rate equation, so that all three are rate-controlling 36 and rate-determining, but the first step is not rate-limiting. 37 Note 2: If the concentration of any intermediate builds up to an appreciable extent, 38 then the steady-state approximation no longer holds, and the reaction should be 39 analyzed as though that intermediate is the reactant.

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1 Note 3: It should be noted that a catalytic cycle does not have a rate-determining 2 step. Instead, under steady-state conditions, all steps proceed at the same rate 3 because the concentrations of all intermediates adjust so as to offset the differences in 4 the corresponding rate constants [360].5 See also Gibbs energy diagram, microscopic diffusion control, mixing control.6 rev[3]7 revGB-revPOC89 rate of reaction

10 v11 (unit: mol dm–3 s–1 or mol L–1 s–1)12 For the general chemical reaction1314 a A + b B = p P + q Q ...1516 occurring under constant-volume conditions, without an appreciable build-up of reaction 17 intermediates, the rate of reaction v is defined as18

19 𝑣 = ― 1𝑎

d[A]d𝑡 = ―

1𝑏

d[B]d𝑡 = ―

1𝑝

d[P]d𝑡 = ―

1𝑞

d[Q]d𝑡

2021 where symbols inside square brackets denote concentrations (conventionally 22 expressed in unit mol dm–3). The symbols R and r are also used instead of v. It is 23 recommended that the unit of time be the second. 24 Example:2526 Cr2O7

2– + 3 (CH3)2CHOH + 8 H+ = 2 Cr3+ + 3 (CH3)2C=O + 7 H2O27

28 𝑣 = ― d[Cr2O 2 ―

7 ]d𝑡 = ―

13

d[(CH3)2CHOH]d𝑡 =

12

d[𝐶𝑟3 + ]d𝑡 =

13

d[(CH3)2C = O]d𝑡

2930 Note: For a stepwise reaction this definition of rate of reaction will apply only if there 31 is no accumulation of intermediate or formation of side products. It is therefore 32 recommended that the term rate of reaction be used only in cases where it is 33 experimentally established that these conditions apply. More generally, it is 34 recommended that, instead, the terms rate of disappearance or rate of consumption of 35 A (i.e., –d[A]/dt) or rate of appearance of P (i.e., d[P]/dt) be used, depending on the 36 particular chemical species that is actually observed. In some cases reference to the 37 chemical flux observed may be more appropriate.38 See [13].

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1 See also chemical relaxation, lifetime, order of reaction.2 rev[3]3 revGB-revPOC45 reaction coordinate6 Parameter that changes during the conversion of one (or more) reactant molecular 7 entities into one (or more) product molecular entities and whose value can be taken as 8 a measure of the progress along a minimum-energy reaction path.9 Note 1: The term "reaction coordinate" is often used to refer to a geometric variable

10 itself (typically a bond distance or bond angle, or a combination of distances and/or 11 angles) as well as (or instead of) the value of that variable. Although strictly incorrect, 12 this usage is very commonly encountered.13 Note 2: In cases where the location of the transition structure is unknown, an internal 14 coordinate of the system (e.g., a geometric variable or a bond order, or an energy gap 15 between reactant-like and product-like valence-bond structures) is often selected as a 16 reaction coordinate. A potential-energy profile obtained by energy minimization over 17 other coordinates for a succession of fixed values of an arbitrary reaction coordinate is 18 not guaranteed to pass through the transition structure unless it is a continuous function 19 of that reaction coordinate. Similarly, a free-energy profile obtained as a potential of 20 mean force with respect to an arbitrary reaction coordinate is not guaranteed to pass 21 through the lowest-energy transition state.22 Note 3: “Reaction coordinate” is sometimes used as an undefined label for the 23 horizontal axis of a potential-energy profile or a Gibbs energy diagram.24 See [349,361]. 25 See also Gibbs energy diagram, potential-energy profile, potential-energy surface.26 rev[3]27 revGB-revPOC2829 reaction path30 (1) Synonym for mechanism.31 (2) Trajectory on the potential-energy surface.32 rev[3]33 revGB-revPOC3435 reaction step36 Elementary reaction constituting one of the stages of a stepwise reaction in which a 37 reaction intermediate (or, for the first step, the reactants) is converted into the next 38 reaction intermediate (or, for the last step, the products) in the sequence of 39 intermediates between reactants and products.40 GB[3]

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12 reactive intermediate3 intermediate45 reactivity (n.), reactive (adj.)6 Kinetic property of a chemical species by which (for whatever reason) it has a different 7 rate constant for a specified elementary reaction than some other (reference) species.8 Note 1: The term has meaning only by reference to some explicitly stated or implicitly 9 assumed set of conditions. It is not to be used for reactions or reaction patterns of

10 compounds in general.11 Note 2: Term also used more loosely as a phenomenological description not 12 restricted to elementary reactions. When applied in this sense, the property under 13 consideration may reflect not only rate constants but also equilibrium constants.14 See also stable, unreactive, unstable.15 GB[3]1617 reactivity index18 Numerical quantity derived from quantum-mechanical model calculations or from a 19 linear Gibbs-energy relationship (linear free-energy relationship) that permits the 20 prediction or correlation of relative reactivities of different molecular sites.21 Note: Many indices are in use, based on a variety of theories and relating to various 22 types of reaction. The more successful applications have been to the substitution 23 reactions of conjugated systems, where relative reactivities are determined largely by 24 changes of -electron energy and -electron density.25 rev[3]26 revGB-revPOC2728 reactivity-selectivity principle (RSP)29 Idea that the more reactive a reagent is, the less selective it is.30 Note: There are many examples in which the RSP is followed, but there are also 31 many counterexamples. Although the RSP is in accord with intuitive feeling, it is now 32 clear that selectivity can decrease, increase, or remain constant as reactivity increases, 33 so that the RSP is unreliable as a guide to reactivity.34 See [126,346,362,363,364,365].35 rev[3]36 revGB-revPOC3738 rearrangement39 See degenerate rearrangement, molecular rearrangement, sigmatropic 40 rearrangement.

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1 GB[3]23 reduction 4 (1) Transfer of one or more electrons to a molecular entity, usually inorganic.5 (2) Decrease in the oxidation number of any atom within any substrate [344].6 (3) Loss of oxygen or halogen and/or gain of hydrogen of an organic substrate.7 See oxidation.8 rev[3]9 revGB-revPOC

1011 reductive elimination12 Reverse of oxidative addition.13 GB[3]1415 regioselectivity (n.), regioselective (adj.)16 Property of a reaction in which one position of bond making or breaking occurs 17 preferentially over all other possible positions.18 Note 1: The resulting regioisomers are constitutional isomers. 19 Note 2: Reactions are termed completely (100 %) regioselective if the discrimination 20 is complete, or partially (x %) if the product of reaction at one site predominates over 21 the product of reaction at other sites. The discrimination may also be referred to semi-22 quantitatively as high or low regioselectivity.23 Note 3: Historically the term was restricted to addition reactions of unsymmetrical 24 reagents to unsymmetrical alkenes.25 Note 4: In the past, the term regiospecificity was proposed for 100 % regioselectivity. 26 This terminology is not recommended, owing to inconsistency with the terms 27 stereoselectivity and stereospecificity.28 See [366,367].29 See also chemoselectivity.30 rev[3]31 revGB-revPOC3233 Reichardt ET parameter34 See Dimroth-Reichardt ET(30) parameter.3536 relaxation37 Passage of a system that has been perturbed from equilibrium, by radiation excitation 38 or otherwise, toward or into thermal equilibrium with its environment.39 See [9].40 See also chemical relaxation.

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1 rev[3]2 revGB-revPOC34 reorganization energy5 Gibbs energy required to distort the reactants (and their associated solvent molecules) 6 from their relaxed nuclear configurations to the relaxed nuclear configurations of the 7 products (and their associated solvent molecules). 8 Note 1: This approach was originally formulated for one-electron transfer reactions, 9 A + D → A–· + D+·, in the framework of the Marcus equation, assuming weak coupling

10 between the reactants [368].11 Note 2: Reorganization energy is not the same as distortion energy, which is the 12 energy required to distort the reactants to the nuclear configuration of the transition 13 state.14 Note 3: This approach has been extended to enzyme-catalysed reactions [369].15 Note 4: Marcus theory has been shown to be valid for some complex reactions 16 (cycloaddition, SN2), even though the weak-coupling assumption is clearly not valid. In 17 these cases the reorganization energy (in terms of activation strain) is counteracted by 18 stabilizing interactions (electrostatic and orbital) [166].19 See also distortion interaction model, intrinsic barrier, Marcus equation.20 rev[3]21 revGB-revPOC2223 resonance24 Representation of the electronic structure of a molecular entity in terms of contributing 25 Lewis structures.26 Note 1: Resonance means that the wavefunction is represented by mixing the 27 wavefunctions of the contributing Lewis structures.28 Note 2: The contributing Lewis structures are represented as connected by a double-29 headed arrow (↔), rather than by the double arrow (⇄) representing equilibrium 30 between species.31 Note 3: This concept is the basis of the quantum-mechanical valence-bond methods. 32 The resulting stabilization is linked to the quantum-mechanical concept of resonance 33 energy. The term resonance is also used to refer to the delocalization phenomenon 34 itself.35 Note 4: This term has a completely different meaning in physics.36 See [75,370].37 See also [371].38 rev[3]39 revGB-revPOC40

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1 resonance effect2 Experimentally observable influence (on reactivity, etc.) of a substituent through 3 electron delocalization to or from the substituent.4 See [126,173,191].5 See also inductive effect.6 rev[3]78 resonance energy9 Difference in potential energy between the actual molecular entity and the contributing

10 Lewis structure of lowest potential energy.11 Note: The resonance energy cannot be measured experimentally, but only 12 estimated, since contributing Lewis structures are not observable molecular entities.13 See resonance.14 rev[3]15 revGB-revPOC1617 resonance form18 resonance structure, contributing structure, canonical form19 One of at least two Lewis structures,with fixed single, double, and triple bonds, that is 20 a contributing structure to the valence-bond wavefunction of a molecule that cannot be 21 described by a single Lewis structure.22 Note 1: Although the valence-bond wavefunction is a linear combination of the 23 wavefunctions of the individual resonance forms, the coefficients and the relative 24 contributions of the various resonance forms are usually kept qualitative. For example, 25 the major resonance forms for the conjugate base of acetone are CH2=C(CH3)–O– and 26 H2C––C(CH3)=O, with the former contributing more.27 Note 2: Resonance forms are connected by a double-headed arrow (↔). This must 28 not be confused with the double arrow connecting species in equilibrium (⇌).29 See also delocalization, Kekulé structure, resonance.3031 resonance hybrid32 Molecular entity whose electronic structure is represented as the superposition of two 33 or more resonance forms (or Lewis structures) with different formal arrangements of 34 electrons but identical arrangements of nuclei.35 Note 1: Whereas each contributing resonance form represents a localized 36 arrangement of electrons which, considered by itself, would imply different bond lengths 37 (say) for formal single and double bonds, resonance between two or more contributors 38 requires each to have the same geometry, namely that of the resulting hybrid, which 39 represents a delocalized arrangement of electrons. A particular bond in the hybrid may 40 have a length that is, loosely, an “average” of formal single-bond and double-bond

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1 values implied by its contributing individual resonance forms, but the hybrid does not 2 oscillate among these as if they were in equilibrium.3 Note 2: The resonance forms are connected by a double-headed arrow (↔), rather 4 than by the double arrow (⇄) representing equilibrium between species.5 diffGB67 retrocycloaddition8 deprecated9 Cycloelimination.

10 [3]1112 GB1314 Ritchie equation15 Linear Gibbs-energy relation (linear free-energy relation)1617 lg {kN} = lg {k0} + N+

1819 applied to the reactions between nucleophiles and certain large and relatively stable 20 organic cations, e.g., arenediazonium, triarylmethyl, and aryltropylium cations, in 21 various solvents, where {kN} is the (reduced) second-order rate constant for reaction of 22 a given cation with a given nucleophilic system (i.e., given nucleophile in a given 23 solvent). {k0} is the (reduced) first-order rate constant for the same cation with water in 24 water, and N+ is a parameter characteristic of the nucleophilic system and independent 25 of the electrophilic reaction partner. 26 The discrepancy between second-order and first-order rate constants must be 27 reconciled by writing the equation with arguments of the logarithms of dimension 1, i.e., 28 by using reduced rate constants (as denoted by the curly brackets in the defining 29 equation):3031 lg [kN/(dm3 mol-1s-1)] = lg (k0/s-1) + N+

3233 Note 1: A surprising feature of the equation is the absence of a coefficient of N+ 34 characteristic of the substrate (cf. the s in the Swain-Scott equation), even though 35 values of N+ vary over 13 decadic log (lg) units. The equation thus involves a gigantic 36 breakdown of the reactivity-selectivity principle.37 Note 2: The Ritchie equation is a special case of the more general Mayr-Patz 38 equation.39 See [193,372,373]. See also [125,374].40 rev[3]

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1 revGB-revPOC 23 -value (rho-value)4 Quantitative measure of the susceptibility of the rate constant or equilibrium constant 5 of an organic reaction to the influence of substituent groups, usually on an aromatic 6 ring.7 Note 1: Defined by Hammett to describe the effects of substituents at the meta- and 8 para-positions on rate or equilibrium of a reaction on the side chain of a substituted 9 benzene, the empirical equation has the general form

1011 lg(kX/kH) or lg(KX/KH) = X

1213 in which X is a constant characteristic of the substituent X and of its position in the 14 reactant molecule.15 Note 2: More generally (not only for aromatic series), -values (modified with 16 appropriate subscripts and superscripts) are used to designate the susceptibility to 17 substituent effects of reactions of families of organic compounds, as given by the 18 modified set of -constants in an empirical correlation.19 Note 3: Reactions with a positive -value are accelerated (or the equilibrium 20 constants are increased) by substituents with positive -constants. Since the sign of 21 was defined so that substituents with a positive increase the acidity of benzoic acid, 22 such substituents are generally described as attracting electrons away from the 23 aromatic ring. It follows that reactions with a positive -value involve a transition state 24 (or reaction product) with an increased electron density at the reactive site of the 25 substrate.26 See also Hammett equation, -constant, Taft equation.27 rev[3]28 revPOC2930 -equation (rho-sigma equation)31 See Hammett equation, -value, -constant, Taft equation.32 GB[3]3334 salt effect35 See kinetic electrolyte effect.36 GB[3]3738 saturation transfer39 See magnetization transfer.40 rev[3]

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1 revGB-revPOC23 Saytzeff rule4 Preferential removal of a hydrogen from the carbon that has the fewest hydrogens in 5 dehydrohalogenation of secondary and tertiary haloalkanes.6 Note 1: The rule was originally formulated by A. Saytzeff (Zaitsev) to generalize the 7 orientation in -elimination reactions of haloalkanes. It has been extended and 8 modified, as follows: When two or more olefins can be produced in an elimination 9 reaction, the thermodynamically most stable alkene will predominate.

10 Note 2: Exceptions to the Saytzeff rule are exemplified by the Hofmann rule.11 See [375].12 See also Markovnikov rule.13 revGB-revPOC[3]1415 scavenger16 Substance that reacts with (or otherwise removes) a trace component (as in the 17 scavenging of trace metal ions) or traps a reactive intermediate.18 See also inhibition.19 GB[3]2021 selectivity22 Discrimination shown by a reagent in competitive attack on two or more substrates or 23 on two or more positions or diastereotopic or enantiotopic faces of the same substrate. 24 Note 1: Selectivity is quantitatively expressed by the ratio of rate constants of the 25 competing reactions, or by the decadic logarithm of such a ratio.26 Note 2: In the context of aromatic substitution (usually electrophilic, for 27 monosubstituted benzene derivatives), the selectivity factor Sf (expressing 28 discrimination between p- and m-positions in PhZ) is defined as 2930 Sf = lg (pf

Z/mfZ)

3132 where the partial rate factors pf

Z and mfZ express the reactivity of para and meta

33 positions in the aromatic compound PhZ relative to that of a single position in benzene.34 See [347].35 See also isoselective relationship, partial rate factor, regioselectivity, 36 stereoselectivity.37 rev[3]38 revGB-revPOC3940 self-assembly

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1 Process whereby a system of single-molecule components spontaneously forms an 2 organized structure, owing to molecular recognition.34 shielding5 Extent to which the effective magnetic field is reduced for a nucleus in a molecule 6 immersed in an external magnetic field, relative to that experienced by a bare nucleus 7 in that field. 8 Note 1: The reduction is due to the circulation of the electrons around the observed 9 and the neighbouring nuclei. The external field induces a magnetic moment that is

10 oriented in the opposite direction to the external field, so that the local field at the central 11 nucleus is weakened, although it may be strengthened at other nuclei (deshielding).12 Note 2: This phenomenon is the origin of the structural dependence of the resonance 13 frequencies of the nuclei.14 See also chemical shift.15 GB[3]1617 shift reagent18 Paramagnetic substance that induces an additional change of the NMR resonance 19 frequency of a nucleus near any site in a molecule to which the substance binds. 20 See chemical shift.2122 sigma, pi23 See , 24 rev[3]25 revGB-revPOC2627 sigmatropic rearrangement28 Molecular rearrangement that involves both the creation of a new bond between 29 atoms previously not directly linked and the breaking of an existing bond. 30 Note 1: There is normally a concurrent relocation of bonds in the molecule 31 concerned, but neither the number of bonds nor the number of bonds changes. 32 Note 2: The transition state of such a reaction may be visualized as an association 33 of two fragments connected at their termini by two partial bonds, one being broken 34 and the other being formed as, for example, the two allyl fragments in (a'). Considering 35 only atoms within the (real or hypothetical) cyclic array undergoing reorganization, if the 36 numbers of these in the two fragments are designated i and j, then the rearrangement

37 is said to be a sigmatropic change of order [i,j] (conventionally i ≤ j). Thus rearrangement 38 (a) is of order [3,3], whereas rearrangement (b) is a [1,5]sigmatropic shift of hydrogen. 39 (By convention the square brackets [...] here refer to numbers of atoms, in contrast with 40 current usage in the context of cycloaddition.)

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123 The descriptors a and s (antarafacial and suprafacial) may also be annexed to the 4 numbers i and j; (b) is then described as a [1s,5s] sigmatropic rearrangement, since it is 5 suprafacial with respect to both the hydrogen atom and the pentadienyl system:6

789 See also cycloaddition, tautomerization.

10 rev[3]11 revGB-revPOC1213 silylene14 (1) Generic name for H2Si: and substitution derivatives thereof, containing an 15 electrically neutral bivalent silicon atom with two non-bonding electrons. (The definition 16 is analogous to that given for carbene.)17 (2) The silanediyl group (H2Si<), analogous to the methylene group (H2C<).18 GB[3]1920 single-electron transfer mechanism (SET)21 Reaction mechanism characterized by the transfer of a single electron between two 22 species, occurring in one of the steps of a multistep reaction.23 GB[3]2425 single-step reaction 26 one-step reaction27 Reaction that proceeds through a single transition state (or no transition state).28 GB[3]29

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12 skeletal formula3 bond-line formula4 Two-dimensional representation of a molecular entity in which bonds are indicated as 5 lines between vertices representing octet carbon atoms with attached hydrogens 6 omitted and in which other atoms are represented by their chemical symbols.7 Examples: ethanol, cyclohexene, 4-pyridone8

91011 See line formula.1213 Slater-type orbital (STO)14 Function centred on an atom for which the radial dependence has the form (r) ∝ rn–1

15 exp(–r), used to approximate atomic orbitals in the LCAO-MO method. 16 Note 1: n is the effective principal quantum number and is the orbital exponent 17 (screening constant) derived from empirical considerations. 18 Note 2: The angular dependence is usually introduced by multiplying the radial 19 function by a spherical harmonic Y1m(,). 20 Note 3: Owing to difficulties in computing the integrals of STOs analytically for 21 molecules with more than two atoms they are often replaced by linear combinations of 22 Gaussian orbitals.23 See [8].24 rev[3]25 revGB-revPOC2627 solvation28 Stabilizing interaction between a solute (or solute moiety) and the solvent. 29 Note: Such interactions generally involve electrostatic forces and van der Waals 30 forces, as well as chemically more specific effects such as hydrogen bond formation. 31 See also cybotactic region.32 GB[3]3334 solvatochromic relationship

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1 Linear Gibbs-energy relationship (linear free-energy relationship) based on 2 solvatochromism. 3 See also Dimroth-Reichardt ET parameter, Kamlet-Taft solvent parameters.4 revGB-revPOC[3]56 solvatochromism7 Pronounced change in position and sometimes intensity of an electronic absorption or 8 emission band, accompanying a change in the polarity of the medium.9 Note: Negative (positive) solvatochromism corresponds to a hypsochromic shift

10 (bathochromic shift) with increasing solvent polarity. 11 See [9,17,376].12 See also Dimroth-Reichardt ET parameter, Z-value.13 GB[3]1415 solvatomers16 Isomers that differ in their solvation environment.17 Note 1: Because the solvation environment fluctuates rapidly, solvatomers 18 interconvert rapidly.19 Note 2: Species that differ in the type of solvent molecules should not be called 20 solvatomers, because they are not isomers.2122 solvent parameter23 Quantity that expresses the capability of a solvent for interaction with solutes, based on 24 experimentally determined physicochemical quantities, in particular: relative 25 permittivity, refractive index, rate constants, Gibbs energies and enthalpies of reaction, 26 and ultraviolet-visible, infrared, and NMR spectra.27 Note 1: Solvent parameters are used in correlation analysis of solvent effects, either 28 in single-parameter or in multiple-parameter equations.29 Note 2: Solvent parameters include those representing a bulk property, such as 30 relative permittivity (dielectric constant) as well as those that describe a more localized 31 solute/solvent interaction, such as hydrogen-bonding acceptance or donation and 32 Lewis acid/base adduct formation. 33 See [17,377].34 See also: acceptor number, Catalán solvent parameters, Dimroth-Reichardt ET 35 parameter, Grunwald-Winstein equation, Kamlet-Taft solvent parameters, Laurence 36 solvent parameters, Koppel-Palm solvent parameters, linear solvation energy 37 relationship, Z-value.38 rev[3]39 revGB-revPOC40

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1 solvolysis2 Reaction with solvent.3 Note 1: Such a reaction generally involves the rupture of one or more bonds in the 4 solute. More specifically the term is used for substitution, elimination, and fragmentation 5 reactions in which a solvent species serves as nucleophile or base. 6 Note 2: A solvolysis can also be classified as a hydrolysis, alcoholysis, or 7 ammonolysis, etc., if the solvent is water, alcohol, or ammonia, etc.8 Note 3: Often a solvolysis is a nucleophilic substitution (usually SN1, accompanied 9 by E1 elimination), where the nucleophile is a solvent molecule.

10 rev[3]11 revGB-revPOC1213 SOMO14 Singly Occupied Molecular Orbital (such as the half-filled HOMO of a radical).15 See also frontier orbitals.16 GB[3]1718 special salt effect19 Steep increase of the rate of certain solvolysis reactions observed at low concentrations 20 of some non-common-ion salts.21 Note: The effect is attributed to trapping of an intimate ion pair that would revert to 22 reactant in the absence of the salt.23 See also kinetic electrolyte effect.24 rev[3]25 revGB-revPOC2627 specific catalysis28 Acceleration of a reaction by a unique catalyst, rather than by a family of related 29 substances. 30 Note: The term is most commonly used in connection with specific hydrogen-ion or 31 hydroxide-ion (lyonium ion or lyate ion) catalysis. 32 See also general acid catalysis, general base catalysis, pseudo-catalysis.33 GB[3]3435 spectator mechanism36 Pre-association mechanism in which one of the molecular entities, C, is already present 37 in an encounter pair with A during formation of B from A, but does not assist the 38 formation of B, e.g.,39

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123 Note: The formation of B from A may itself be a bimolecular reaction with some other 4 reagent. Since C does not assist the formation of B, it is described as being present as 5 a spectator.6 See also microscopic diffusion control.7 GB[3]89 spin adduct

10 See spin trapping.11 GB[3]1213 spin counting14 See spin trapping.15 GB[3]1617 spin density18 Unpaired electron density at a position of interest, usually at carbon, in a radical or a 19 triplet state.20 Note: Spin density is often measured experimentally by electron paramagnetic 21 resonance/electron spin resonance (EPR/ ESR) spectroscopy through hyperfine 22 splitting of the signal by neighbouring magnetic nuclei.23 See also radical centre.24 GB[3]2526 spin label27 Stable paramagnetic group (typically an aminoxy radical, R2NO) that is attached to a 28 part of a molecular entity whose chemical environment may be revealed by its electron 29 spin resonance (ESR) spectrum.30 Note: When a paramagnetic molecular entity is used without covalent attachment to 31 the molecular entity of interest, it is frequently referred to as a spin probe.32 GB[3]3334 spin trapping35 Formation of a more persistent radical from interaction of a transient radical with a 36 diamagnetic reagent.37 Note 1: The product radical accumulates to a concentration where detection and, 38 frequently, identification are possible by EPR/ESR spectroscopy.

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1 Note 2: The key reaction is usually one of attachment; the diamagnetic reagent is 2 said to be a spin trap, and the persistent product radical is then the spin adduct. The 3 procedure is referred to as spin trapping, and is used for monitoring reactions involving 4 the intermediacy of reactive radicals at concentrations too low for direct observation. 5 Typical spin traps are C-nitroso compounds and nitrones, to which reactive radicals will 6 rapidly add to form aminoxy radicals. 7 Example:

89 Note 3: A quantitative development in which essentially all reactive radicals

10 generated in a particular system are intercepted has been referred to as spin counting.11 Note 4: Spin trapping has also been adapted to the interception of radicals generated 12 in both gaseous and solid phases. In these cases the spin adduct is in practice 13 transferred to a liquid solution for EPR/ESR observation.14 GB[3]1516 stable17 Having a lower standard Gibbs energy, compared to a reference chemical species.18 Note 1: Quantitatively, in terms of Gibbs energy, a chemical species A is more stable 19 than its isomer B if ∆rG° is positive for the (real or hypothetical) reaction A → B. 20 Note 2: For the two reactions2122 P → X + Y ∆rG1°23 and Q → X + Z ∆rG2°2425 if ∆rG1° > ∆rG2°, then P is more stable relative to its product Y than is Q relative to Z.26 Note 3: Both in qualitative and quantitative usage the term stable is therefore always 27 used in reference to some explicitly stated or implicitly assumed standard.28 Note 4: The term should not be used as a synonym for unreactive or less reactive 29 since this confuses thermodynamics and kinetics. A relatively more stable chemical 30 species may be more reactive than some reference species towards a given reaction 31 partner.32 See also inert, unstable.33 GB[3]3435 stationary state

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1 (1) (in quantum mechanics): Wavefunction whose probability density | |2 remains 2 constant and whose observable properties do not evolve with time.3 (2) (in kinetics): See steady state.4 GB[3]56 steady state (or stationary state)7 (1) Approximation that the kinetic analysis of a complex reaction involving unstable 8 intermediates in low concentration can be simplified by setting the rate of change of 9 each such intermediate equal to zero, so that the rate equation can be expressed as a

10 function of the concentrations of chemical species present in macroscopic amounts.11 For example, if X is an unstable intermediate in the reaction sequence:

121314 Since [X] is negligibly small, d[X]/dt, the rate of change of [X], can be set equal to zero. 15 The steady state approximation then permits solving the following equation1617 d[X]/dt = k1[A] – k–1[X] – k2[X][C] = 01819 to obtain the steady-state [X]:2021 [X] = k1[A]/(k–1 + k2[C])2223 whereupon the rate of reaction is expressed:2425 d[D]/dt = k2[X][C] = k1k2[A][C]/(k–1 + k2[C])2627 Note: The steady-state approximation does not imply that [X] is even approximately 28 constant, only that its absolute rate of change is very much smaller than that of [A] and 29 [D].30 (2) Regime in a stirred flow reactor such that all concentrations are independent of time.31 See [13].32 GB[3]3334 stepwise reaction35 Chemical reaction with at least one reaction intermediate and involving at least two 36 consecutive elementary reactions.37 See also composite reaction, reaction step.38 GB[3]

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12 stereoelectronic3 Pertaining to the dependence of the properties (especially energy or reactivity) of a 4 molecular entity or of a transition state on the relative disposition of electron pairs owing 5 to the nuclear geometry. 6 Note: Stereoelectronic effects are ascribed to the differing overlaps of atomic orbitals 7 in different conformations.8 See [11].9 GB[3]

1011 stereogenic centre12 Atom within a molecule bearing groups such that interchanging any two of them leads 13 to a stereoisomer of the original molecule. 14 See [11,378].1516 stereoisomers17 Isomers that have the same bonds (connectivity) but differ in the arrangement of their 18 atoms and cannot be interconverted by rapid rotation around single bonds.19 See [11].20 GB2122 stereoselectivity (stereoselective)23 Preferential formation in a chemical reaction of one stereoisomer over another.24 Note 1: When the stereoisomers are enantiomers, the phenomenon is called 25 enantioselectivity and is quantitatively expressed by the enantiomeric excess or 26 enantiomeric ratio; when they are diastereoisomers, it is called diastereoselectivity and 27 is quantitatively expressed by the diastereomeric excess or diastereomeric ratio. 28 Note 2: Reactions are termed 100 % stereoselective if the preference is complete, 29 or partially (x %) stereoselective if one product predominates. The preference may also 30 be referred to semiquantitatively as high or low stereoselectivity31 See [11,140].32 revGB[3]3334 stereospecificity (stereospecific)35 Property of those chemical reactions in which different stereoisomeric reactants are 36 converted into different stereoisomeric products.37 Example: electrocyclization of trans,cis,trans-octa-2,4,6-triene produces cis-5,6-38 dimethylcyclohexa-1,3-diene, whereas cis,cis,trans-octa-2,4,6-triene produces racemic 39 trans-5,6-dimethylcyclohexa-1,3-diene.40

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123 Note 1: A stereospecific process is necessarily stereoselective but not all 4 stereoselective processes are stereospecific. Stereospecificity may be total (100 %) or 5 partial.6 Note 2: The term is also applied to situations where reaction can be performed with 7 only one stereoisomer. For example the exclusive formation of racemic trans-1,2-8 dibromocyclohexane upon bromination of cyclohexene is a stereospecific process, 9 even though the analogous reaction with (E)-cyclohexene has not been performed.

10 Note 3: Stereospecificity does NOT mean very high stereoselectivity. This usage is 11 unnecessary and is strongly discouraged.12 See [11,140].13 For the term stereospecific polymerization see [379].14 rev[3]15 revGB1617 steric-approach control18 Situation in which the stereoselectivity of a reaction under kinetic control is governed 19 by steric hindrance to attack of the reagent, which is directed to the less hindered face 20 of the molecule.21 Note: Partial bond making at the transition state must be strong enough for steric 22 control to take place, but the transition state should not be so close to products that the 23 steric demand of the reagent at the transition state is the same as the steric demand of 24 the group as present in the product.25 An example is LiAlH4 reduction of 3,3,5-trimethylcyclohexan-1-one, where steric 26 hindrance by an axial methyl directs hydride addition to the equatorial position, even 27 though the more stable product has the H axial and the OH equatorial.28 See also product-development control.29 rev[3]30 revGB31

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1234 steric effect5 Consequences for molecular geometry, thermochemical properties, spectral features, 6 solvation, or reaction rates resulting from the fact that atoms repel each other at close 7 distance. The repulsion is due to the quantum-mechanical Pauli exclusion principle. 8 Substitution of hydrogen atoms by groups with a larger van der Waals radius may lead 9 to situations where atoms or groups of atoms repel each other, thereby affecting

10 distances and angles.11 Note 1: It is in principle difficult to separate the steric effect from other electronic 12 effects.13 Note 2: For the purpose of correlation analysis or linear Gibbs-energy relations 14 (linear free-energy relations) various scales of steric parameters have been proposed, 15 notably A values, Taft's ES and Charton's v scales.16 Note 3: A steric effect on a rate process may result in a rate increase (steric 17 acceleration) or a decrease (steric retardation) depending on whether the transition 18 state or the reactant state is more affected by the steric effect.19 Note 4: Bulky groups may also attract each other if at a suitable distance.20 See [290,380]. 21 See Taft equation, van der Waals forces.22 rev[3]23 revGB-revPOC2425 steric hindrance26 Steric effect whereby the crowding of substituents around a reaction centre retards the 27 attack of a reagent.28 rev[3]29 revGB-revPOC3031 stopped flow32 Technique for following the kinetics of reactions in solution (usually in the millisecond 33 time range) in which two, or more, reactant solutions are rapidly mixed by being forced 34 through a mixing chamber. The flow of the mixed solution along a uniform tube is then 35 suddenly arrested. At a fixed position along the tube the solution is monitored as a 36 function of time following the stoppage of the flow by a method with a rapid response 37 (e.g., optical absorption spectroscopy).38 See mixing control.39 rev[3]40 revGB-revPOC

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12 strain3 Feature of a molecular entity or transition structure for which the energy is increased 4 because of unfavourable non-bonded (steric) interactions, bond lengths, bond angles, 5 or dihedral angles (torsional strain), relative to a standard.6 Note 1: The strain energy is quantitatively defined as the standard enthalpy of a 7 structure relative to that of a strainless structure (real or hypothetical) made up from the 8 same atoms with the same types of bonding. 9 For example, the enthalpy of formation of cyclopropane is +53.6 kJ mol-1, whereas

10 the hypothetical enthalpy of formation based on three "normal" methylene groups, from 11 acyclic models, is -62 kJ mol-1. On this basis cyclopropane is destabilized by ca. 115 kJ 12 mol–1 of strain energy. 13 See molecular mechanics.14 GB[3]1516 structural isomers17 discouraged term for constitutional isomers.1819 subjacent orbital20 Next-to-Highest Occupied Molecular Orbital (NHOMO, also called HOMO–1). 21 Note: Subjacent and superjacent orbitals sometimes play an important role in the 22 interpretation of molecular interactions according to the frontier orbital approach. 23 See [381].24 GB[3]2526 substituent27 Any atom or group of bonded atoms that can be considered to have replaced a 28 hydrogen atom (or two hydrogen atoms in the special case of bivalent groups) in a 29 parent molecular entity (real or hypothetical).30 GB[3]3132 substitution33 Chemical reaction, elementary or stepwise, of the form A–B + C → A–C + B, in which 34 one atom or group in a molecular entity is replaced by another atom or group. 35 Examples36 CH3Cl + HO– → CH3OH + Cl–

37 C6H6 + NO2+ → C6H5NO2 + H+

38 Note: A substitution reaction can be distinguished as an electrophilic substitution or 39 a nucleophilic substitution, depending on the nature of the reactant that is considered 40 to react with the substrate.

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1 rev[3]2 revGB-revPOC34 substrate5 Chemical species, the reaction of which with some other chemical reagent is under 6 observation (e.g., a compound that is transformed under the influence of a catalyst).7 Note: The term should be used with care. Either the context or a specific statement 8 should always make it clear which chemical species in a reaction is regarded as the 9 substrate.

10 See also transformation.11 GB[3]1213 successor complex14 Chemical species formed by the transfer of an electron or of a hydrogen (atom or ion) 15 from a donor D to an acceptor A after these species have diffused together to form the 16 precursor or encounter complex:

1718 rev[3]19 revGB-revPOC2021 suicide inhibition22 See mechanism-based inhibition.23 GB[3]2425 superacid26 Medium having a high acidity, generally greater than that of 100 % sulfuric acid. The 27 common superacids are made by dissolving a powerful Lewis acid (e.g., SbF5) in a 28 suitable Brønsted acid, such as HF or HSO3F.29 Note 1: An equimolar mixture of HSO3F and SbF5 is known by the trade name Magic 30 Acid.31 Note 2: An uncharged gas-phase substance having an endothermicity (enthalpy) of 32 deprotonation (dehydronation) lower than that of H2SO4 is also called a superacid. 33 Nevertheless such a superacid is much less acidic in the gas phase than similar cationic 34 acids.35 See [18,382,383,384].36 See acidity, superbase.37 rev[3]38 revGB-revPOC39

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1 superbase2 Compound having a very high basicity.3 Examples include amide bases such as LDA, potassium tert-butoxide + 4 organolithium, some phosphazenes.5 See superacid.6 See [385].7 rev[3]8 revGB-revPOC9

10 superjacent orbital11 Second Lowest Unoccupied Molecular Orbital (SLUMO). 12 Note: Subjacent and superjacent orbitals sometimes play an important role in the 13 interpretation of molecular interactions according to the frontier orbital approach. 14 See [381].1516 suprafacial17 See antarafacial.18 GB[3]1920 supramolecular21 Description of a system of two or more molecular entities that are held together and 22 organized by means of intermolecular (noncovalent) binding interactions.23 See [386].24 revGB[3]2526 Swain-Lupton equation27 Dual-parameter approach to the correlation analysis of substituent effects, which 28 involves a field constant (F) and a resonance constant (R).2930 lg(kX/kH) = fFX + rRX

3132 See [191,387].33 Note 1: The original treatment was modified later.34 Note 2: The procedure has often been applied, but also often criticized.35 See [388,389,390,391,392].36 revGB-revPOC[3]3738 Swain-Scott equation39 Linear Gibbs-energy relation (linear free-energy relation) of the form40

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1 lg(k/k0) = sn23 applied to the variation of reactivity of a given electrophilic substrate towards a series 4 of nucleophilic reagents, where k0 is a rate constant for reaction with water, k is the 5 corresponding rate constant for reaction with any other nucleophilic reagent, n is a 6 measure of the nucleophilicity of the reagent (n = 0.0 for water) and s is a measure of 7 the sensitivity of the substrate to the nucleophilicity of the reagent (s = 1.0 for CH3Br). 8 See [192].9 See also Mayr-Patz equation, Ritchie equation.

10 GB[3]1112 symproportionation13 comproportionation14 GB[3]1516 syn17 See anti.18 See [11].19 GB[3]2021 synartetic acceleration22 See neighbouring group participation.23 GB[3]2425 synchronization26 See principle of nonperfect synchronization.27 rev[3]28 GB2930 synchronous31 Feature of a concerted process in which all the changes (generally bond rupture and 32 bond formation) have progressed to the same extent at the transition state. 33 Note 1: A synchronous reaction is distinguished from (1) a concerted reaction, which 34 takes place in a single kinetic step without being synchronous, (2) a reaction where 35 some of the changes in bonding take place earlier, followed by the rest, and (3) a two-36 step reaction, which takes place in two kinetically distinct steps, via a stable 37 intermediate 38 Note 2: The progress of the bonding changes (or other primitive changes) is not 39 defined quantitatively in terms of a single parameter applicable to different bonds. The 40 concept therefore does not admit an exact definition except in the case of concerted

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1 processes involving changes in two identical bonds. If the bonds are not identical, the 2 process should simply be described as concerted.3 See [393,394,395]. For an index of synchronicity see [396].4 See also imbalance.5 rev[3]6 GB78 , 9 Symmetry designations that distinguish molecular orbitals as being symmetric () or

10 antisymmetric () with respect to a defining plane containing at least one atom. 11 Note 1: In practice the terms are used both in this rigorous sense (for orbitals 12 encompassing the entire molecule) and for localized two-centre orbitals or bonds. In 13 the case of localized two-centre bonds, a bond has a nodal plane that includes the 14 internuclear bond axis, whereas a bond has no such nodal plane. (A bond in 15 organometallic or inorganic chemical species has two nodes.) Radicals are classified 16 by analogy into and radicals.17 Note 2: Such two-centre orbitals may take part in molecular orbitals of or 18 symmetry. For example, the methyl group in propene contains three C–H bonds, each 19 of which is of local symmetry (i.e., without a nodal plane including the internuclear 20 axis), but these three bonds can in turn be combined to form a set of group orbitals 21 one of which has symmetry with respect to the principal molecular plane and can 22 accordingly interact with the two-centre orbital of symmetry ( bond) of the double-23 bonded carbon atoms, to form a molecular orbital of symmetry. Such an interaction 24 between the CH3 group and the double bond is an example of hyperconjugation. This 25 cannot rigorously be described as - conjugation since and here refer to different 26 defining planes, and interaction between orbitals of different symmetries (with respect 27 to the same defining plane) is forbidden.28 Note 3: Conjugation between a system and a lone pair of (for example) an ether 29 oxygen involves a lone pair of symmetry with respect to the defining plane. It is 30 incorrect to consider this as an interaction between the system and one of two 31 identical sp3-hybrid lone pairs, which are neither nor .32 Note 4: The two lone pairs on a carbonyl oxygen are properly classified as or 33 with respect to the plane perpendicular to the molecular plane (dotted line perpendicular 34 to the plane of the page), rather than as two sp2-hybridized lone pairs. This distinction 35 readily accounts for the facts that there are two different lone-pair ionization energies 36 and two different n-* excited states.37

38

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12345 See also [397].6 revGB-revPOC78 -adduct9 Product formed by the attachment of an electrophilic or nucleophilic entering group or

10 of a radical to a ring carbon of an aromatic species so that a new bond is formed and 11 the original conjugation is disrupted.12 Note 1: This has generally been called a complex, or a Wheland complex from 13 electrophilic addition, but adduct is more appropriate.14 Note 2: The term may also be used for analogous adducts to systems.15 See also Meisenheimer adduct.16 rev[3]17 GB1819 -constant 20 Substituent constant for meta- and para-substituents in benzene derivatives as defined 21 by Hammett on the basis of the ionization constant of a substituted benzoic acid, i.e., 22 lg(Ka

X/KaH), where Ka

X is the ionization constant (acid-dissociation constant) of a m- or 23 p-substituted benzoic acid and Ka

H that of benzoic acid itself.24 Note 1: A large positive -value implies high electron-withdrawing power by an 25 inductive and/or resonance effect, relative to H; a large negative -value implies high 26 electron-releasing power relative to H.27 Note 2: The term is also used as a collective description for related electronic 28 substituent constants based on other standard reaction series, of which, +,– and o 29 are typical; also for constants which represent dissected electronic effects, such as I 30 and R. For example,– (sigma-minus) constants are defined on the basis of the 31 ionization constants of para-substituted phenols, where such substituents as nitro show 32 enhanced electron-withdrawing power.33 See [124,125,126,191,226,398]. 34 See also Hammett equation, -value, Taft equation.35 GB[3]3637 Taft equation38 Linear free-energy relation (linear Gibbs-energy relation) involving the polar substituent 39 constant * and the steric substituent constant Es, as derived from reactivities of 40 aliphatic esters

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12 lg {k} = lg {k0} + ρ*σ* + Es

34 The argument of the lg function should be of dimension 1. Thus, the reduced rate 5 constants should be used, i.e., the rate coefficient divided by its units: {k} = k/[k] and 6 {k0} = k0/[k0].7 Note: The standard reaction (k0) is the hydrolysis of methyl acetate, whereby Es is 8 evaluated from the rate of acid-catalyzed hydrolysis, relative to that of methyl acetate, 9 and * is evaluated from the ratio of the rates of base- and acid-catalyzed hydrolysis.

10 See [172,380,399,400]. 11 See also Hammett equation, -value, -constant.12 rev[3]13 revGB-revPOC1415 tautomers16 Constitutional isomers that can interconvert more or less rapidly, often by migration of 17 a hydron.18 See tautomerization.1920 tautomerization21 Rapid isomerization of the general form22 G–X–Y=Z ⇌ X=Y–Z–G23 where the isomers (called tautomers) are readily interconvertible.24 Note 1: The atoms of the groups X,Y, Z are typically any of C, H, N, O, or S, and G 25 is a group that becomes an electrofuge or nucleofuge during isomerization.26 Note 2: The commonest case, when the electrofuge is H+, is also known as a 27 prototropic rearrangement.28 Examples:29

3031

Commented [s2]: Alphabetical order, tautomers is later.

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1 Note 3: The group Y may itself be a three-atom (or five-atom) chain extending the 2 conjugation, as in3

456 Note 4: Ring-chain tautomerization is the case where addition across a double bond 7 leads to ring formation, as in8

910 Note 5: Valence tautomerization is the case of rapid isomerization involving the 11 formation and rupture of single and/or double bonds, without migration of atoms: for 12 example

1314 See [346,401].15 See also ambident, fluxional, sigmatropic rearrangement, valence tautomerization.16 rev[3]17 revGB-revPOC18 tautomers19 Constitutional isomers that can interconvert more or less rapidly, often by migration of 20 a hydron.21 See tautomerization.2223 tele-substitution24 Substitution reaction in which the entering group takes up a position more than one 25 atom away from the atom to which the leaving group was attached.26 Example27

28

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1 See also cine-substitution.2 See [402,403].3 rev[3]4 revGB-revPOC56 termination7 Step(s) in a chain reaction in which reactive intermediates are destroyed or rendered 8 inactive, thus ending the chain.9 GB[3]

1011 tetrahedral intermediate12 Reaction intermediate in which the bond arrangement around an initially double-bonded 13 carbon atom (typically a carbonyl carbon) has been transformed from tricoordinate to 14 tetracoordinate (with a coordination number of 4).15 Example:

1617 rev[3]18 revGB-revPOC192021 thermodynamic control (of product composition)22 equilibrium control23 Conditions (including reaction times) that lead to reaction products in a proportion 24 specified by the equilibrium constant for their interconversion.25 See also kinetic control.26 rev[3]27 revGB-revPOC2829 thermolysis30 Uncatalysed cleavage of one or more covalent bonds resulting from exposure of a 31 molecular entity to a raised temperature, or a process of which such cleavage is an 32 essential part.33 See also pyrolysis.34 GB[3]3536 through-conjugation37 Phenomenon whereby electrons can be delocalized from any of three (or more) groups 38 to any other.

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1 Example: p-XC6H4Y, where an electron pair can be delocalized from electron-2 donating X not only to ring carbons but also to electron-withdrawing Y. 3 Note 1: This may be contrasted with cross-conjugation.4 Note 2: In Hammett-type correlations (linear Gibbs-energy relationships) this 5 situation can lead to exalted substituent constants + or –, as in solvolysis of p-6 CH3OC6H4C(CH3)2Cl or acidity of p-nitrophenol, respectively. 78 TICT9 See twisted intramolecular charge transfer.

1011 torquoselectivity12 Preference for inward or outward rotation of substituents in conrotatory or disrotatory 13 electrocyclic ring-opening and ring-closing reactions, often owing to a preference for 14 electron donors (especially including large groups) to rotate outward and acceptors to 15 rotate inward.16 See [404,405].

1718 rev[3]19 revGB2021 transferability22 Assumption that a chemical property associated with an atom or group of atoms in a 23 molecule will have a similar (but not identical) value in other circumstances.24 Examples: equilibrium bond length, bond force constant, NMR chemical shift.25 GB[3]26

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1 transformation2 Conversion of a substrate into a particular product, irrespective of the specific reagents 3 or mechanisms involved.4 Example: transformation of aniline (C6H5NH2) into N-phenylacetamide 5 (C6H5NHCOCH3), which may be effected with acetyl chloride or acetic anhydride or 6 ketene.7 Note: A transformation is distinct from a reaction, the full description of which would 8 state or imply all the reactants and all the products.9 See [406].

10 GB[3]1112 transient (chemical) species13 Short-lived reaction intermediate.14 Note 1: Transiency can be defined only in relation to a time scale fixed by the 15 experimental conditions and by the limitations of the technique employed in the 16 detection of the intermediate. The term is a relative one.17 Note 2: Transient species are sometimes also said to be metastable. However, this 18 latter term should be avoided, because it relates a thermodynamic term to a kinetic 19 property, although most transients are also thermodynamically unstable with respect to 20 reactants and products.21 See also persistent.22 GB[3]2324 transition dipole moment25 Vector quantity describing the oscillating electronic moment induced by an 26 electromagnetic wave, given by an integral involving the dipole moment operator m and 27 the ground- and excited-state wavefunctions:2829 M = exc m gnd d3031 Note: The magnitude of this quantity describes the allowedness of an electronic 32 transition. It can often be separated into an electronic factor and a nuclear-overlap factor 33 known as the Franck-Condon factor.34 See [9].3536 transition state37 State of a molecular system from which there are equal probabilities of evolving toward 38 states of lower energy, generally considered as reactants and products of an 39 elementary reaction.

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1 Note 1: The transition state corresponds to the maximum along the minimum-Gibbs-2 energy path connecting reactants and products.3 Note 2: The transition state can be considered to be a chemical species of transient 4 existence.5 Note 3: The term transition-state structure refers to a structure inferred from kinetic 6 and stereochemical investigations; it does not necessarily coincide with a transition 7 state or with a transition structure, which corresponds to a saddle point on a potential-8 energy surface, although it may represent an average structure.9 Note 4: The assembly of atoms at the transition state has also been called an

10 activated complex, although it is not a complex according to the definition in this 11 Glossary.12 Note 5: There are also reactions, such as the gas-phase colligation of simple radicals 13 or the reactions of some reactive intermediates in solution, that do not require activation 14 and do not involve a transition state.15 Note 6: Ultrafast spectroscopy permits observation of transition states in some 16 special cases.17 See [349,407].18 See also Gibbs energy of activation, potential-energy profile, reaction coordinate, 19 transition structure.20 rev[3]21 revGB-revPOC2223 transition-state analogue24 Species designed to mimic the geometry and electron density of the transition state of 25 a reaction, usually enzymatic.26 Note: A transition-state analogue is usually not a substrate for the enzyme, but rather 27 an inhibitor.28 rev[3]29 revGB-revPOC3031 transition structure 32 Molecular entity corresponding to a saddle point on a potential-energy surface, with one 33 negative force constant and its associated imaginary frequency.34 Note 1: Whereas the transition state is not a specific molecular structure, but a set 35 of structures between reactants and products, a transition structure is one member of 36 that set with a specific geometry and energy.37 Note 2: Although the saddle point coincides with the potential-energy maximum 38 along a minimum-energy reaction path, it does not necessarily coincide with the 39 maximum of Gibbs energy for an ensemble of chemical species.40 Note 3: The term transition-state structure is not a synonym for transition structure.

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1 See [349,408].2 See also activated complex, transition state.3 rev[3]4 revGB-revPOC56 transition vector7 Normal mode of vibration of a transition structure corresponding to the single imaginary 8 frequency and tangent to the intrinsic reaction coordinate at the saddle point. 9 Sometimes called the reaction-coordinate vibrational mode.

10 Note: Infinitesimal motion along the transition vector in the two opposite senses 11 determines the initial direction leading either toward reactants or toward products.12 Note 2: The term "transition coordinate" was used in the 1994 Glossary of Terms in 13 Physical Organic Chemistry in the sense defined here for transition vector, but 14 apparently this usage was unique in the literature at that time and has not been 15 generally adopted since.16 See [349,409].17 See reaction coordinate, transition state.18192021 transport control22 See encounter-controlled rate, microscopic diffusion control.23 GB[3]2425 trapping26 Interception of a reactive molecule or reaction intermediate so that it is removed from 27 the system or converted into a more stable form for study or identification.28 See also scavenger.29 GB[3]3031 triplet state32 State having a total electron spin quantum number of 1.33 See [9]. 3435 tunnelling36 Quantum-mechanical phenomenon by which a particle or a set of particles penetrates 37 a barrier on its potential-energy surface without having the energy required to surmount 38 that barrier.

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1 Note 1: In consequence of Heisenberg's Uncertainty Principle, a molecular entity has 2 a nonzero probability of adopting a geometry that is classically forbidden because it 3 corresponds to a potential energy greater than the total energy.4 Note 2: Tunnelling is often considered to be a correction to an (over)simplified 5 version of transition-state theory.6 Note 3: Because the rate of tunnelling increases with decreasing mass, it is 7 significant in the context of isotope effects, especially of hydrogen isotopes.8 See [410,411].9 rev[3]

10 revGB-revPOC1112 twisted intramolecular charge transfer (TICT)13 Feature of an excited electronic state formed by intramolecular electron transfer from 14 an electron donor (D) to an electron acceptor (A), where interaction between electron 15 and hole is restricted because D+ and A– are perpendicular to each other. 16 See [412].1718 umpolung19 Process by which the nucleophilic or electrophilic property of a functional group is 20 reversed.21 Note: Umpolung is often achieved by temporary exchange of heteroatoms N or O by 22 others, such as P, S, or Se, as in the conversion of electrophilic RCH=O to RCH(SR')2 23 and then with base to nucleophilic RC(SR')2

–. Also the transformation of a haloalkane 24 RX into a Grignard reagent RMgCl is an umpolung.25 See [413].26 rev[3]27 revGB-revPOC2829 unimolecular30 Feature of a reaction in which only one molecular entity is involved.31 See molecularity.32 rev[3]33 revGB-revPOC3435 unreactive36 Failing to react with a specified chemical species under specified conditions.37 Note: The term should not be used in place of stable, which refers to a 38 thermodynamic property, since a relatively more stable species may nevertheless be 39 more reactive than some reference species towards a given reaction partner.40 GB[3]

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12 unstable3 Opposite of stable, i.e., the chemical species concerned has a higher molar Gibbs 4 energy than some assumed standard.5 Note: The term should not be used in place of reactive or transient, although more 6 reactive or transient species are frequently also more unstable.7 rev[3]8 revGB-revPOC9

10 upfield11 superseded but still widely used to mean shielded.12 See chemical shift.13 GB[3]1415 valence16 Maximum number of single bonds that can be commonly formed by an atom or ion of 17 the element under consideration.18 Note: Often there is a most common maximum for a given element, and atoms in 19 compounds where this number is exceeded, such as pentacoordinate carbocations 20 (“carbonium ions”) and iodine(III) compounds, are called hypervalent.21 rev[3]22 revGB-revPOC 2324 valence isomer25 Constitutional isomer related to another by pericyclic reaction.26 Examples: Dewar benzene, prismane, and benzvalene are valence isomers of 27 benzene.28 Note: Valence isomers are separable, as distinguished from valence tautomers, 29 which interconvert rapidly. See valence tautomerization.30 rev[3]31 revGB-revPOC3233 valence tautomerization34 Rapid isomerization involving the formation and rupture of single and/or double bonds, 35 without migration of atoms.36 Example:

3738 See tautomerization.

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1 rev[3]2 revGB-revPOC34 van der Waals forces5 Attractive or repulsive forces between molecular entities (or between groups within the 6 same molecular entity) other than those due to bond formation or to the electrostatic 7 interaction of ions or of ionic groups with one another or with neutral molecules.8 Note 1: The term includes dipole-dipole, dipole-induced dipole, and London 9 (instantaneous induced dipole-induced dipole) forces, as well as quadrupolar forces.

10 Note 2: The term is sometimes used loosely for the totality of nonspecific attractive 11 or repulsive intermolecular forces.12 Note 3: In the context of molecular mechanics, van der Waals forces correspond only 13 to the London dispersion forces plus the Pauli repulsion forces. Interactions due to the 14 average charge distribution (and not to fluctuations around the average or to induced 15 dipoles), including dipole-dipole, ion-ion and ion-dipole forces, are called Coulombic 16 forces.17 See [290,414].18 See also dipole-dipole interaction, dipole-induced dipole forces, London forces.19 rev[3]20 revGB-revPOC2122 volume of activation, ∆‡V23 Quantity derived from the pressure dependence of the rate constant of a reaction, 24 defined by the equation2526 ∆‡V = RT T[∂ln(𝑘/[𝑘])/∂𝑝]2728 provided that rate constants of all reactions (except first-order reactions) are expressed 29 in pressure-independent concentration units, such as mol dm–3 at a fixed temperature 30 and pressure. The argument in the lg function should be of dimension 1. Thus, the rate 31 constant should be divided by its units, [k].32 Note: The volume of activation is interpreted as the difference between the partial 33 molar volume ‡V of the transition state and the sums of the partial molar volumes of the 34 reactants at the same temperature and pressure, i.e.,3536 ∆‡V = ‡V – (VR)3738 where is the order in the reactant R and VR its partial molar volume.39 See [13].40 See order of reaction.

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1 rev[3]2 revGB-revPOC34 water/octanol partition coefficient (partition ratio)5 See octanol-water partition ratio67 wavefunction 8 A mathematical expression whose form resembles the wave equations of physics, 9 supposed to contain all the information associated with a particular atomic or molecular

10 system. In particular, a solution of the Schrödinger wave equation, H = E, as an 11 eigenfunction of the hamiltonian operator H, which involves the electronic and/or 12 nuclear coordinates.13 Note 1: The wavefunction contains all the information describing an atomic or 14 molecular system that is consistent with the Heisenberg Uncertainty Principle.15 Note 2: When a wavefunction is operated on by certain quantum-mechanical 16 operators, a theoretical evaluation of physical and chemical observables for that system 17 (the most important one being energy) can be carried out.18 See [8,12]. 19 Expanded GB2021 Wheland intermediate22 See Meisenheimer adduct, -adduct.23 GB[3]2425 Woodward-Hoffmann rules26 See orbital symmetry.27 GB28293031 ylide32 Chemical species that can be produced by loss of a hydron from an atom directly 33 attached to the central atom of an onium ion.34 Example:35 Ph3P+–CHRR’ → [Ph3P+–C–RR’ ↔ Ph3P=CRR’] + H+

36 See [40].37 diffGB[3]3839 Yukawa-Tsuno equation

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1 Multiparameter extension of the Hammett equation to quantify the role of enhanced 2 resonance effects on the reactivity of para–substituted benzene derivatives, 34 lg {k} = lg {ko} + [o + r(+ –o)]5 or lg {k} = lg {ko} + [σo + r(– –o)]67 where the parameter r expresses the enhancement and where o is a substituent 8 constant based on reactivities of phenylacetic acids and similar substrates, where 9 resonance interaction is weak or absent. The argument in the lg function should be of

10 dimension 1. Thus, reduced rate constants should be used {k}= k/[k] and {ko} = ko/[ko].11 See [398,415,416].12 See also dual substituent-parameter equation, -value, -constant, through-13 conjugation.14 rev[3]15 revGB-revPOC1617 Zaitsev rule18 See Saytzeff rule.19 GB[3]2021 zero-point energy22 Extent, in consequence of Heisenberg's Uncertainty Principle, by which a particle or a 23 set of particles has an energy greater than that of the minimum on the potential-energy 24 surface.25 Note 1: Because of zero-point energy a molecular entity has a nonzero probability of 26 adopting a geometry whose energy is greater than that of the energy minimum. 27 Note 2: A molecular entity with zero-point energy may even adopt a geometry with 28 a potential energy greater than its total energy, a possibility that permits tunnelling.29 Note 3: Because the magnitude of zero-point energy increases with decreasing 30 mass, it is significant in the context of isotope effects, especially of hydrogen isotopes.3132 Zucker-Hammett hypothesis33 Assumption that if lg {k1} (= k1/[k1], reduced pseudo-first-order rate constant of an acid-34 catalyzed reaction) is linear in Ho (Hammett acidity function), then water is not involved 35 in the transition state of the rate-controlling step, whereas if lg {k1} is linear in lg {[H+]} 36 then water is involved. The argument in the lg function should be of dimension 1. Thus, 37 reduced concentration = {[H+]} should be used, i.e., concentration of protons divided by 38 its units.39 Note: This has been shown to be an overinterpretation.40 See [22,417].

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1 See also Bunnett-Olsen equation, Cox-Yates equation.2 rev[3]3 revGB-revPOC45 Z-value6 Quantitative measure of solvent polarity based on the UV-vis spectrum of 1-ethyl-4-7 (methoxycarbonyl)pyridinium iodide.8 See [143].9 See solvent parameter.

10 rev[3]11 revGB-revPOC1213 zwitterion14 Highly dipolar, net uncharged (neutral) molecule having full electrical charges of 15 opposite sign, which may be delocalized within parts of the molecule but for which no 16 uncharged canonical resonance structure can be written.

17 Examples: glycine (H3N+–CH2–CO2−), betaine (Me3N+–CH2–CO2

−).18 Note 1: Sometimes also referred to as inner salts or ampholytes.19 Note 2: Mesoionic compounds, such as sydnones, in which both positive and 20 negative charge are delocalized, are sometimes considered as zwitterions, but species 21 with a localized nonzero formal charge, such as a nitrone, CH3CH=N+(–O–)CH3, are 22 not.23 See [418].24 rev[3]25 revGB-revPOC26

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GLOSSARY OF TERMS USED IN 1

PHYSICAL ORGANIC CHEMISTRY 2 3 4 Abstract. This Glossary contains definitions, explanatory notes, and sources for terms 5 used in physical organic chemistry. Its aim is to provide guidance on the terminology of 6 physical organic chemistry, with a view to achieving a consensus on the meaning and 7 applicability of useful terms and the abandonment of unsatisfactory ones. Owing to the 8 substantial progress in the field, this 2021 revision of the Glossary is much expanded 9 relative to the previous edition, and it includes terms from cognate fields. 10 11 12

INTRODUCTION TO THE 2021 REVISION 13 General Remarks 14

15 The first Glossary of Terms Used In Physical Organic Chemistry was published 16 in provisional form in 1979 [1] and in revised form in 1983, incorporating modifications 17 agreed to by IUPAC Commission III.2 (Physical Organic Chemistry) [2]. 18 A further revision was undertaken under the chairmanship of Paul Müller, which 19 was published in 1994 [3]. The work was coordinated with that of other Commissions 20 within the Division of Organic Chemistry. In 1999 Gerard P. Moss, with the assistance 21 of Charles L. Perrin, converted this glossary to a World Wide Web version [4]. The 22 Compendium of Chemical Terminology [5] (Gold Book) incorporated many of the terms 23 in the later version. 24

This Glossary has now been thoroughly revised and updated, to be made 25 available as a Web document. The general criterion adopted for the inclusion of a term 26 in this Glossary has been its wide use in the present or past literature of physical-27 organic chemistry and related fields, with particular attention to those terms that have 28 been ambiguous. It is expected that the terms in this Glossary will be incorporated within 29 the on-line version of the IUPAC Gold Book, which is the merged compendium of all 30 glossaries [5]. 31

The aim of this Glossary is to provide guidance on the terminology of physical-32 organic chemistry, with a view to achieving a far-reaching consensus on the definitions 33 of useful terms and the abandonment of unsatisfactory ones. According to Antoine 34 Lavoisier "Comme ce sont les mots qui conservent les idées et qui les transmettent, il 35 en résulte qu'on ne peut perfectionner le langage sans perfectionner la science, ni la 36 science sans le langage,“ (As it is the words that preserve the ideas and convey them, 37 it follows that one cannot improve the language without improving science, nor improve 38

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2

science without improving the language.”) [6]. Our approach has been to take or update 1 entries from the previous glossary, whereas new terms were added by virtue of their 2 usage in the current literature and the diverse knowledge of the members of the Task 3 Force. 4

The Task Force is pleased to acknowledge the generous contributions of many 5 scientists who helped by proposing or defining new terms or by criticizing or modifying 6 existing ones. The following members of the Task Force have contributed to this 7 revision: 8 9

Israel Agranat Charles L. Perrin, Chair 10 Alessandro Bagno† Leo Radom 11 Silvia E. Braslavsky Zvi Rappoport 12 Pedro A. Fernandes Marie-Françoise Ruasse† 13 Jean-François Gal Hans-Ullrich Siehl 14 Guy C. Lloyd-Jones Yoshito Takeuchi 15 Herbert Mayr Thomas T. Tidwell 16 Joseph R. Murdoch Einar Uggerud 17 Norma Sbarbati Nudelman Ian H. Williams 18

19 († = deceased) 20 This version was prepared for publication by Charles L. Perrin, Silvia E. Braslavsky, 21 and Ian Williams. We especially note the able technical assistance of Gerard P. Moss 22 (Queen Mary University of London) and the advice of Christian Reichardt (Marburg). 23 We also thank the several reviewers whose comments improved the presentation, 24 especially Jan Kaiser (University of East Anglia). 25 26 Arrangement, Abbreviations, and Symbols 27 The arrangement is alphabetical, terms with Greek letters following those in the 28 corresponding Latin ones. Italicized words in the body of a definition, as well as those 29 cited at the end, point to relevant cross-references. Literature references direct the 30 reader either to the original literature where the term was originally defined or to 31 pertinent references where it is used, including other IUPAC glossaries, where 32 definitions may differ from those here. 33 Definitions of techniques not directly used for measurements in physical-organic 34 chemistry are not included here but may be consulted in specialized IUPAC texts, 35 including the IUPAC Recommendations for Mass Spectrometry [7], the Glossary of 36 Terms Used In Theoretical Organic Chemistry [8], the Glossary of Terms Used in 37 Photochemistry 3rd edition [9], the Glossary of Terms Used in Photocatalysis and 38 Radiation Catalysis [10] and the Basic Terminology of Stereochemistry [11]. 39

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In accordance with IUPAC recommendations [12] the symbol ‡ to indicate 1 transition state ("double dagger") is used as a prefix to the appropriate quantities, e.g., 2 ∆‡G rather than ∆G‡. In equations including a logarithmic function the procedure 3 recommended by IUPAC was adopted, i.e., to divide each dimensioned quantity by its 4 units. Since this procedure often introduces a cluttering of the equations, we have in 5 some cases chosen a short-hand notation, such as ln {k(T)}, where the curly brackets 6 indicate an argument of dimension one, corresponding to k(T)/[k(T)], where the square 7 brackets indicate that the quantity is divided by its units, as recommended in [12]. 8 9 Note on the identification of new and/or revised terms. 10 Terms that are found in the previous version of this Glossary [3,4] and currently 11 incorporated in the IUPAC “Gold Book” [5] are identified with a reference to the Glossary 12 of Terms used in Physical Organic Chemistry (1994), i.e., [3], whereas revised terms 13 are designated as rev[3]. Minor changes such as better wording, additional cross-14 referencing, or reorganization of the text without changing the concept are, in general, 15 not considered revisions. However, the improved version should replace the older one 16 in the “Gold Book”. New terms and terms from other IUPAC documents are not 17 identified as such. In many cases new references have been added in the definitions. 18 19 20

21

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A factor 1 Arrhenius factor 2 (SI unit same as rate constant: s–1 for first-order reaction). 3 Pre-exponential factor in the Arrhenius equation for the temperature dependence of a 4 reaction rate constant. 5

Note 1: According to collision theory, A is the frequency of collisions with the correct 6 orientation for reaction. 7

Note 2: The common unit of A for second-order reactions is dm3 mol-1 s-1. 8 See also A value, energy of activation, entropy of activation. 9 See [12,13]. 10 rev[3] 11

12 A value 13 Steric substituent parameter expressing the conformational preference of an equatorial 14 substituent relative to an axial one in a monosubstituted cyclohexane. 15

Note 1: This parameter equals DrGº for the equatorial to axial equilibration, 16 in kJ mol–1. For example, ACH3 is 7.28 kJ mol–1, a positive value because an axial 17

methyl group is destabilized by a steric effect. 18 Note 2: The values are also known as Winstein-Holness A values. 19 See [3,14,15]. 20

21 abstraction 22 Chemical reaction or transformation, the main feature of which is the bimolecular 23 removal of an atom (neutral or charged) from a molecular entity. 24

Examples: 25 CH3COCH3 + (i-C3H7)2N– → [CH3COCH2]– + (i-C3H7)2NH 26 (hydron abstraction from acetone) 27 CH4 + Cl·!→!CH3· + HCl 28

(hydrogen atom abstraction from methane) 29 See detachment. 30 [3] 31

32 acceptor parameter (A) 33 acceptor number (deprecated). Unit and dimension 1. 34 Quantitative measure, devised by Gutmann [16] of the Lewis acidity of a solvent, based 35 on the 31P chemical shift of dissolved triethylphosphine oxide (triethylphosphane oxide). 36

Note: The term acceptor number, designated by AN, is a misnomer and ought to be 37 called acceptor parameter, A, because it is an experimental value [17]. 38

rev[3] 39 40

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acid 1 Molecular entity or chemical species capable of donating a hydron (proton) or capable 2 of forming a bond with the electron pair of a Lewis base. 3

See Brønsted acid, Lewis acid, Lewis base. 4 See also hard acid. 5 [3] 6

7 acidity 8 (1) Of a compound: 9 Tendency of a Brønsted acid to act as a hydron (proton) donor, or tendency of a Lewis 10 acid to form Lewis adducts and p-adducts. 11

Note: Acidity can be quantitatively expressed by the acid dissociation constant of the 12 compound in water or in some other specified medium, by the association constants for 13 formation of Lewis adducts and p-adducts, or by the enthalpy or Gibbs energy of 14 deprotonation in the gas phase. 15 (2) Of a medium, usually one containing Brønsted acids: 16 Tendency of the medium to hydronate a specific reference base. 17

Note 1: The acidity of a medium is quantitatively expressed by the appropriate acidity 18 function. 19

Note 2: Media having an acidity greater than that of 100 % H2SO4 are often called 20 superacids. 21

See [18]. 22 rev[3] 23

24 acidity function 25 Measure of the thermodynamic hydron-donating or -accepting ability of a solvent 26 system, or a closely related thermodynamic property, such as the tendency of the lyate 27 ion of the solvent system to form Lewis adducts. 28

Note 1: Acidity functions are not unique properties of the solvent system alone but 29 depend on the solute (or family of closely related solutes) with respect to which the 30 thermodynamic tendency is measured. 31

Note 2: Commonly used acidity functions are extensions of pH to concentrated acidic 32 or basic solutions. Acidity functions are usually established over a range of 33 compositions of such a system by UV/Vis spectrophotometric or NMR measurements 34 of the degree of hydronation (or Lewis adduct formation) for the members of a series of 35 structurally similar indicator bases (or acids) of different strength: the best known of 36 these is the Hammett acidity function Ho (for primary aromatic amines as indicator 37 bases). 38

For detailed information on other acidity functions, on excess acidity, on the 39 evaluation of acidity functions, and on the limitations of the concept, see [19,20,21,22]. 40

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[3] 1 2 activated complex 3

See activated state. 4 rev[3] 5

6 activated state 7 In theories of unimolecular reactions an energized chemical species, often 8 characterized by the superscript ‡, where the excitation is specific and the molecule is 9 poised for reaction. 10

Note 1: Often used as a synonym for activated complex or transition state, but not 11 restricted to transition-state theory. 12

Note 2: This is distinct from an energized molecule, often characterized by the 13 superscript *, in which excitation energy is dispersed among internal degrees of 14 freedom. 15

Note 3: This is not a complex according to the definition in this Glossary. 16 See also transition state, transition structure. 17

18 activation energy 19

See energy of activation. 20 See [12], section 2.12. 21 [3] 22

23

activation strain model 24 See distortion interaction model. 25

26 addition reaction 27 Chemical reaction of two or more reacting molecular entities, resulting in a product 28 containing all atoms of all components, with formation of two chemical bonds and a net 29 reduction in bond multiplicity in at least one of the reactants. 30

Note 1: The addition to an entity may occur at only one site (1,1-addition, insertion), 31 at two adjacent sites (1,2-addition) or at two non-adjacent sites (1,3- or 1,4-addition, 32 etc.). 33

Examples 34 35

R3C–H + H2C: → R3C–CH3 (1,1-addition of a carbene) 36 Br2 + CH2=CH–CH=CH2!→ BrCH2–CH(Br)–CH=CH2 (1,2-addition) + 37

BrCH2–CH=CH–CH2Br (1,4-addition) 38 39

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Note 2: This is distinguished from adduct formation, which is less specific about 1 bonding changes. 2

Note 3: The reverse process is called an elimination reaction. 3 See also cheletropic reaction, cycloaddition, insertion. 4 rev[3] 5

6 additivity principle 7 Hypothesis that each of several structural features of a molecular entity makes an 8 independent, transferable, and additive contribution to a property of the substance 9 concerned. 10

Note 1: More specifically, it is the hypothesis that each of the several substituent 11 groups in a parent molecule makes a separate and additive contribution to the standard 12 Gibbs energy change or Gibbs energy of activation corresponding to a particular 13 chemical reaction. 14

Note 2: The enthalpies of formation of series of compounds can be described by 15 additivity schemes [23]. 16

Note 3: Deviations from additivity may be remedied by including terms describing 17 interactions between atoms or groups. 18

See transferability [24,25]. 19 rev[3] 20

21 adduct 22 New chemical species AB, each molecular entity of which is formed by direct 23 combination of two (or more) separate molecular entities A and B in such a way that 24 there is no loss of atoms from A or B. 25

Example: adduct formed by interaction of a Lewis acid with a Lewis base: 26 (CH3)3N+––BF3 27 Note 1: Stoichiometries other than 1:1 are also possible, e.g., a bis-adduct (2:1). 28 Note 2: An intramolecular adduct can be formed when A and B are groups contained 29

within the same molecular entity. 30 Note 3: If adduct formation is prevented by steric hindrance, frustrated Lewis pairs 31

may result. 32 See also addition reaction, frustrated Lewis pair, Lewis adduct, Meisenheimer 33

adduct, p-adduct. 34 rev[3] 35

36 agostic 37 Feature of a structure in which a hydrogen atom is bonded to both a main-group atom 38 and a metal atom. 39

Example, [(1–3-h)-but-2-en-1-yl-h2-C4,H4]( h5-cyclopentadienyl)cobalt(+1). 40

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1 2 3 4 5

Note: The expression h-hydrido-bridged is also used to describe the bonding 6 arrangement with a bridging hydrogen, but this usage is deprecated. 7

See [26,27,28,29]. 8 rev[3] 9

10 alcoholysis 11 Reaction with an alcohol solvent. 12

Examples: 13 14

C3H5(OCOR)3 (triglyceride) + 3 CH3OH → C3H5(OH)3 (glycerol) + 3 RCOOH 15 (CH3)3CCl + CH3OH → (CH3)3COCH3 + H+ + Cl– 16

17 See solvolysis. 18 rev[3] 19 20

21 allotropes 22 Different structural modifications of an element, with different bonding arrangements of 23 the atoms. 24

Examples for carbon include diamond, fullerenes, graphite, and graphene. 25 See [29]. 26

27 allylic substitution reaction 28 Substitution reaction on an allylic system with leaving group (nucleofuge) at position 1 29 and double bond between positions 2 and 3. The incoming group may become attached 30 to atom 1, or else the incoming group may become attached at position 3, with 31 movement of the double bond from 2,3 to 1,2. 32

Example: 33 CH3CH=CHCH2Br + HOAc → CH3CH=CHCH2OAc or CH3CH(OAc)CH=CH2 34

35 Note: This term can be extended to systems such as: 36

37 CH3CH=CH–CH=CH–CH2Br → CH3CH(OAc)–CH=CH–CH=CH2 + 38

CH3CH=CH–CH(OAc)–CH=CH2 + CH3CH=CH–CH=CH–CH2OAc 39 40

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and also to propargylic substitution, with a triple bond between positions 2 and 3 and 1 possible rearrangement to an allenic product. 2

rev[3] 3 4 alternant 5 Property of a conjugated system of p electrons whose atoms can be divided into two 6 sets (marked as "starred" and "unstarred") so that no atom of either set is directly linked 7 to any other atom of the same set. 8

Examples 9 10 11 12 13 14 15 16

Note 1: According to several approximate theories (including HMO theory), the p 17 MOs for an alternant hydrocarbon are paired, such that for an orbital of energy a + xb 18 there is another of energy a - xb. The coefficients of paired molecular orbitals at each 19 atom are the same, but with opposite sign for the unstarred atoms, and the p electron 20 density at each atom in a neutral alternant hydrocarbon is unity. 21

See [30,31,32]. 22 rev[3] 23

24 ambident 25 ambidentate, bidentate 26 Characteristic of a chemical species whose molecular entities each possess two 27 alternative, distinguishable, and strongly interacting reactive centres, to either of which 28 a bond may be made. 29

Note 1: Term most commonly applied to conjugated nucleophiles, for example an 30 enolate ion (which may react with electrophiles at either the a-carbon atom or the 31 oxygen) or a 4-pyridone, and also to the vicinally ambident cyanide ion and to cyanate 32 ion, thiocyanate ion, sulfinate ions, nitrite ion, and unsymmetrical hydrazines. 33 34

35 36

Note 2: Ambident electrophiles are exemplified by carboxylic esters RC(O)OR', 37 which react with nucleophiles at either the carbonyl carbon or the alkoxy carbon, and 38

C CO

C CO– α

–

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by Michael acceptors, such as enones, that can react at either the carbonyl or the a-1 carbon. 2

Note 3: Molecular entities containing two non-interacting (or feebly interacting) 3 reactive centres, such as dianions of dicarboxylic acids, are not generally considered 4 to be ambident or bidentate and are better described as bifunctional. 5

Note 4: The Latin root of the word implies two reactive centres, but the term has also 6 been applied to chemical species with more than two reactive centres, such as an acyl 7 thiourea, RCONHCSNHR', with nucleophilic O, S, and N. For such species the term 8 polydentate (or multidentate) is more appropriate. 9

See [33,34,35,36]. 10 [3] 11

12 ambiphilic 13 Both nucleophilic and electrophilic. 14

Example: 15 CH2=O 16

See also amphiphilic, but the distinction between ambi (Latin: both) and amphi 17 (Greek: both) and the application to hydrophilic and lipophilic or to nucleophilic and 18 electrophilic is arbitrary. 19 20 aminoxyl 21 Compound having the structure 22

R2N–O•!!«!!"2N•+–O– 23

Note: The synonymous term 'nitroxyl radical' erroneously suggests the presence of 24 a nitro group; its use is deprecated. 25 26 amphiphilic 27 Both hydrophilic and lipophilic, owing to the presence in the molecule of a large organic 28 cation or anion and also a long hydrocarbon chain (or other combination of polar and 29 nonpolar groups, as in nonionic surfactants) 30

Examples: 31 32

CH3(CH2)nCO2–M+, CH3(CH2)nSO3–M+, CH3(CH2)nN(CH3)3+X– (n > 7). 33 34

Note: The presence of distinct polar (hydrophilic) and nonpolar (hydrophobic) regions 35 promotes the formation of micelles in dilute aqueous solution. 36

See also ambiphilic. 37 [3] 38

39 amphiprotic (solvent) 40

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Feature of a self-ionizing solvent possessing characteristics of both Brønsted acid and 1 base. 2

Examples: H2O, CH3OH. 3 [3] 4

5 amphoteric 6 Property of a chemical species that can behave as either an acid or as a base. 7

Examples: H2O, HCO3– (hydrogen carbonate) 8 Note: This property depends upon the medium in which the species is investigated. 9

For example HNO3 is an acid in water but becomes a base in H2SO4. 10 rev[3] 11

12 anchimeric assistance 13 neighbouring group participation 14

[3] 15 16 anionotropy 17 Rearrangement or tautomerization in which the migrating group moves with its electron 18 pair. 19

See [37]. 20 [3] 21

22 annelation 23 Alternative, but less desirable term for annulation. 24

Note: The term is widely used in German and French languages. 25 [3] 26

27 annulation 28 Transformation involving fusion of a new ring to a molecule via two new bonds. 29

Note: Some authors use the term annelation for the fusion of an additional ring to an 30 already existing one, and annulation for the formation of a ring from an acyclic 31 precursor. 32

See [38,39]. 33 See also cyclization. 34 [3] 35

36 annulene 37 Conjugated monocyclic hydrocarbon of the general formula CnHn (n even) with the 38 maximum number of noncumulative double bonds and without side chains. 39

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Note: In systematic nomenclature an annulene may be named [n]annulene, where n 1 is the number of carbon atoms, e.g., [8]annulene for cycloocta-1,3,5,7-tetraene. 2

See [40]. 3 See aromatic, Hückel (4n + 2) rule. 4 [3] 5

6 anomeric effect 7 Tendency of an electronegative substituent alpha to a heteroatom in a six-membered 8 ring to prefer the axial position, as in the anomers of glucopyranose. 9

Note 1: The effect can be generalized to the conformational preference of an 10 electronegative substituent X to be antiperiplanar to a lone pair of atom Y in a system 11 R–Y–C–X with geminal substituents RY and X. 12

Note 2: The effect can be attributed, at least in part, to n-s* delocalization of the lone 13 pair on Y into the C–X s* orbital. 14

See [11,41,42,43,44]. 15 16 anomers 17 The two stereoisomers (epimers) of a cyclic sugar or glycoside that differ only in the 18 configuration at C1 of aldoses or C2 of ketoses (the anomeric or acetal/ketal carbon). 19

Example: 20 21 22 23 24

See [11]. 25 [3] 26

27 antarafacial, suprafacial 28 Two spatially different ways whereby bonding changes can occur when a part of a 29 molecule undergoes two changes in bonding (bond-making or bond-breaking), either 30 to a common centre or to two related centres external to itself. These are designated 31 as antarafacial if opposite faces of the molecule are involved, and suprafacial if both 32 changes occur at the same face. The concept of face is clear from the diagrams in the 33 cases of planar (or approximately planar) frameworks with interacting p orbitals. 34

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1 2 3 4 5 6 7 8 For examples of the use of these terms see cycloaddition, sigmatropic rearrangement. 9

See also anti, s, p. 10 rev[3] 11

12 anti 13 Stereochemical relationship of two substituents that are on opposite sides of a 14 reference plane, in contrast to syn, which means "on the same side". 15

Note 1: Two substituents attached to atoms joined by a single bond are anti if the 16 torsion angle (dihedral angle) between the bonds to the substituents is greater than 90º, 17 in contrast to syn if it is less than 90º. 18

Note 2: A further distinction is made between antiperiplanar, synperiplanar, anticlinal 19 and synclinal. 20

See [11,45,46,47]. 21 Note 3: When the terms are used in the context of chemical reactions or 22

transformations, they designate the relative orientation of substituents in the substrate 23 or product: 24

(1) Addition to a carbon-carbon double bond: 25 26 27 28 29 30 31 32 33 34 35

(2) Alkene-forming elimination: 36 37

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1 2

(3) Aldol reaction (where syn and anti designate the relative orientation of CH3 3 and OH in the product) 4 5

6 Note 4: In examples (1) and (2) anti processes are always antarafacial, and syn 7

processes are suprafacial. 8 Note 5: In the older literature the terms anti and syn were used to designate 9

stereoisomers of oximes and related compounds. That usage was superseded by the 10 terms trans and cis or E and Z. 11

See [11]. 12 rev[3] 13

14 anti-Hammond effect 15 If a structure lying off the minimum-energy reaction path (MERP) is stabilized, the 16 position of the transition state moves toward that structure. 17

See Hammond Postulate, More O'Ferrall - Jencks diagram, perpendicular effect. 18 rev[3] 19

20 aprotic (solvent) 21 non-HBD solvent (non-Hydrogen-Bond Donating solvent) 22 Solvent that is not capable of acting as a hydrogen-bond donor. 23

Note: Although this definition applies to both polar and nonpolar solvents, the 24 distinction between HBD and non-HBD (or between protic and aprotic) is relevant only 25 for polar solvents. 26

See HBD solvent. 27 rev[3] 28

29 30

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aquation 1 Incorporation of one or more integral molecules of water into a species, with or without 2 displacement of one or more other atoms or groups. 3

Example: The incorporation of water into the inner ligand sphere of an inorganic 4 complex. 5

See [48]. 6 See also hydration. 7 [3] 8

9 aromatic (adj.), aromaticity (n.) 10 (1) Having a chemistry typified by benzene (traditionally). 11 (2) Feature of a cyclically conjugated molecular entity whose electronic energy is 12 significantly lower or whose stability is significantly greater (owing to delocalization) 13 than that of a hypothetical localized structure (e.g., Kekulé structure). 14

Note 1: If the molecular entity is of higher energy or less stable than a hypothetical 15 localized structure, the entity is said to be antiaromatic. 16

Note 2: A geometric parameter indicating bond-length equalization has been used 17 as a measure of aromatic character, as expressed in the harmonic oscillator model of 18 aromaticity [49]. 19

Note 3: The magnitude of the magnetically induced ring current, as observed 20 experimentally by NMR spectroscopy or by the calculated nucleus-independent 21 chemical shift (NICS) value, is another measure of aromaticity [50]. 22

Note 4: The terms aromatic and antiaromatic have been extended to describe the 23 stabilization or destabilization of transition states of pericyclic reactions. The 24 hypothetical reference structure is here less clearly defined, and use of the term is 25 based on application of the Hückel (4n + 2) rule and on consideration of the topology of 26 orbital overlap in the transition state, whereby a cycle with (4n+2) electrons and a 27 Möbius cycle with 4n electrons are aromatic. Reactions of molecules in the ground state 28 involving antiaromatic transition states proceed much less easily than those involving 29 aromatic transition states. 30

See [51,52,53]. See 19 articles in [54]. 31 See also Hückel (4n + 2) rule, Möbius aromaticity. 32 [3] 33

34 Arrhenius equation 35 Empirical expression for the temperature dependence of a reaction rate constant k as 36 k(T) = A exp(–EA/RT), 37 with A the pre-exponential factor (Arrhenius A factor) and EA the Arrhenius energy of 38 activation, both considered to be temperature-independent. 39

rev[3] 40

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1 aryne 2 Hydrocarbon derived from an arene by formal removal of two vicinal hydrogen atoms. 3

Example: 1,2-didehydrobenzene (benzyne) 4 5

6 7

Note 1: 1,4-Didehydrobenzene ("p-benzyne diradical", structure above right). 8 Despite common usage this is not an aryne because there is no triple bond, and the 9 usage is deprecated. 10

Note 2: Arynes are usually transient species. 11 Note 3: The analogous heterocyclic compounds are called heteroarynes or 12

hetarynes. 13 See [40,55]. 14 rev[3] 15

16 association 17 Assembling of separate molecular entities into any aggregate, especially of oppositely 18 charged free ions into ion pairs or larger and not necessarily well-defined clusters of 19 ions held together by electrostatic attraction. 20

Note: The term signifies the reverse of dissociation, but is not commonly used for the 21 formation of definite adducts by colligation or coordination. 22

[3] 23 24 asymmetric induction 25 Preferential formation in a chemical reaction of one enantiomer or diastereoisomer over 26 the other as a result of the influence of a chiral centre (stereogenic centre, chiral feature) 27 in the substrate, reagent, catalyst, or environment. 28

Note: The term also refers to the formation of a new chiral centre or chiral feature 29 preferentially in one configuration under such influence. 30

See [11]. 31 rev[3] 32

33 atomic charge 34 Net charge due to the nucleus and the average electronic distribution in a given region 35 of space. This region is considered to correspond to an atom in a molecular entity. 36

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Note 1: The boundary limits of an atom in a polyatomic molecular entity cannot be 1 defined, as they are not a quantum-mechanical observable. Therefore, different 2 conceptual schemes of dividing a molecule into individual atoms will result in different 3 atomic charges. 4

Note 2: The atomic charge on an atom should not be confused with its formal charge. 5 For example, the N in NH4+ is calculated to carry a net negative charge even though its 6 formal charge is +1, and each H is calculated to carry a net positive charge even though 7 its formal charge is 0. 8

See [8]. 9 10 atomic orbital 11 Wavefunction that depends explicitly on the spatial coordinates of only one electron 12 around a single nucleus. 13

See also molecular orbital. 14 See [8]. 15 [3] 16

17 atropisomers 18 Stereoisomers that are enantiomeric owing to hindered rotation about a single bond. 19

Example (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl, BINAP): 20 21 22 23 24 25 26 27

See [11]. 28

29 attachment 30 Transformation by which one molecular entity (the substrate) is converted into another 31 by the formation of one (and only one) two-centre bond between the substrate and 32 another molecular entity and which involves no other changes in connectivity in the 33 substrate. 34

Example: formation of an acyl cation by attachment of carbon monoxide to a 35 carbenium ion (R+): 36

R+ + CO → (RCO)+ 37 See also colligation. 38 rev[3] 39

40

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autocatalytic reaction 1 Chemical reaction in which a product (or a reaction intermediate) also functions as 2 catalyst. 3

Note: In such a reaction the observed rate of reaction is often found to increase with 4 time from its initial value. 5

Example: acid-catalyzed bromination of acetophenone, PhCOCH3, because the 6 reaction generates HBr, which functions as a catalyst. 7

rev[3] 8 9 automerization 10 degenerate rearrangement 11

[3] 12 13 autoprotolysis 14 Proton transfer reaction (hydron transfer reaction) between two identical amphoteric 15 molecules (usually of a solvent), one acting as a Brønsted acid and the other as a 16 Brønsted base. 17

Example: 18 2 H2O → H3O+ + HO– 19 [3] 20

21 autoprotolysis constant 22 Product of the activities of the species produced as the result of autoprotolysis. 23

Example: The autoprotolysis constant for water, Kw (or KAP), is equal to the product 24 of the relative activities of the hydronium and hydroxide ions at equilibrium in pure water. 25 26

Kw = a(H3O+)a(HO–) = 1.0 ´ 10–14 at 25 °C and 1 standard atmosphere. 27 28

Note: Since the relative activity a(H2O) of water at equilibrium is imperceptibly 29 different from unity (with mole fraction as the activity scale and pure un-ionized water 30 as the standard state), the denominator in the expression for the thermodynamic 31 equilibrium constant Kw° for autoprotolysis has a value very close to 1. 32 33

Kw° =a!H3O+"a!HO–"

a!H2O"2 34

35 Furthermore, owing to the low equilibrium extent of dissociation, and if infinite dilution 36 is selected as the standard state, the activity coefficients of the hydronium and 37 hydroxide ions in pure water are very close to unity. This leads to the relative activities 38 of H3O+ and HO- being virtually identical with the numerical values of their molar 39

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concentrations, if the molar scale of activity is used and an activity of 1 mol dm-3 is 1 chosen as the standard state. Thus 2 3

Kw » [H3O+][HO-] 4 5 where [H3O+] and [HO-] are the numerical values of the molar concentrations. The 6 autoprotolysis constant has the unit (and dimension) 1 because each relative activity 7 has dimension 1, being a quotient of an absolute activity (including units) divided by a 8 common unit standard state activity (including units). 9 See [56,57]. 10

rev[3] 11 12 a (alpha) 13 (1) Designation applied to the carbon to which a functional group is attached. 14 (2) In carbohydrate nomenclature a stereochemical designation of the configuration at 15 the anomeric carbon. 16 (3) Parameter in a Brønsted relation expressing the sensitivity of the rate of protonation 17 to acidity. 18 (4) Parameter in Leffler's relation expressing the sensitivity of changes in Gibbs 19 activation energy to changes in overall Gibbs energy for an elementary reaction. 20

See [11]. 21 rev[3] 22

23 a-effect 24 Positive deviation of an a nucleophile (one bearing an unshared pair of electrons on an 25 atom adjacent to the nucleophilic site) from a Brønsted-type plot of lg {knuc} vs. pKa. The 26 argument in the lg function should be of dimension 1. Thus, reduced rate coefficients 27 should be used. Here {knuc} = knuc/[knuc] is the reduced knuc. 28

Note 1: More generally, it is the influence on the reactivity at the site adjacent to the 29 atom bearing a lone pair of electrons. 30

Note 2: The term has been extended to include the effect of any substituent on an 31 adjacent reactive centre, e.g., the a-silicon effect. 32

See [58,59,60]. 33 See also Brønsted relation. 34 rev[3] 35

36 a-elimination 37 1,1-elimination 38 Transformation of the general type 39 40

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RR'ZXY → RR'Z + XY (or X + Y, or X+ + Y–) 1 2 where the central atom Z is commonly carbon. 3

See also elimination. 4 rev[3] 5

6 Baldwin's rules 7 Set of empirical rules for closures of 3- to 7-membered rings. 8

Note: The favoured pathways are those in which the length and nature of the linking 9 chain enable the terminal atoms to achieve the proper geometry and orbital overlaps 10 for reaction. 11

Example: Hex-5-en-1-yl radical undergoes 5-exo-trig cyclization to cyclopentylmethyl 12 radical rather than 6-endo-trig cyclization to cyclohexyl radical. 13 14

15 16

See [61,62]. 17 rev[3] 18

19 base 20 Chemical species or molecular entity having an available pair of electrons capable of 21 forming a bond with a hydron (proton) (see Brønsted base) or with the vacant orbital of 22 some other species (see Lewis base). 23

See also hard base, superbase. 24 [3] 25

26 basicity 27 (1) Tendency of a Brønsted base to act as hydron (proton) acceptor. 28

Note 1: The basicity of a chemical species is normally expressed by the acidity or 29 acid-dissociation constant of its conjugate acid (see conjugate acid-base pair). 30

Note 2: To avoid ambiguity, the term pKaH should be used when expressing basicity 31 by the acid-dissociation constant of its conjugate acid. Thus the pKaH of NH3 is 9.2, 32 while its pKa, expressing its acidity, is 38. 33 (2) Tendency of a Lewis base to act as a Lewis acid acceptor. 34

Note 3: For Lewis bases, basicity is expressed by the association constants of Lewis 35 adducts and p-adducts, or by the enthalpy of an acid/base reaction. 36

Note 4: Spectroscopic shifts induced by acid/base adduct formation can also be used 37 as a measure of the strength of interaction. 38

See [63]. 39

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rev[3] 1 2 bathochromic shift (effect) 3 Shift of a spectral band to lower frequencies (longer wavelengths). 4

Note: This is informally referred to as a red shift and is opposite to a hypsochromic 5 shift ("blue shift"), but these historical terms are discouraged because they apply only 6 to visible transitions. 7

See [9]. 8 [3] 9

10 Bell-Evans-Polanyi principle 11 Linear relation between energy of activation (EA) and enthalpy of reaction (DrH), 12 sometimes observed within a series of closely related reactions. 13 14 EA = a + b DrH 15 16

See [64,65,66,67]. 17 rev[3] 18

19 benzyne 20 1,2-Didehydrobenzene (a C6H4 aryne derived from benzene) and its derivatives formed 21 by substitution. 22

Note: The terms m- and p-benzyne are occasionally used for 1,3- and 1,4-23 didehydrobenzene, respectively, but these are incorrect because there is no triple bond. 24

See [40,55]. 25 [3] 26

27 bidentate 28 Feature of a ligand with two potential binding sites. 29 30 bifunctional catalysis 31 Catalysis (usually of hydron transfer) by a chemical species involving a mechanism in 32 which two functional groups are implicated in the rate-limiting step, so that the 33 corresponding catalytic coefficient is larger than that expected for catalysis by a 34 chemical species containing only one of these functional groups. 35

Example: Hydrogen carbonate is a particularly effective catalyst for the hydrolysis of 36 4-hydroxybutyranilide (N-phenyl-4-hydroxybutanamide), because it catalyzes the 37 breakdown of the tetrahedral intermediate to expel aniline: 38 39

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1 2

Note: The term should not be used to describe a concerted process involving the 3 action of two different catalysts. 4

See [68,69,70,71,72]. 5 rev[3] 6

7 bifurcation 8 Feature on a potential-energy surface whereby a minimum-energy reaction path 9 (MERP) emanating from a saddle point (corresponding to a transition structure) splits 10 in two and leads to alternative products without intervening minima or secondary 11 barriers to overcome. A bifurcation arises when the curvature of the surface in a 12 direction perpendicular to the MERP becomes zero and then negative; it implies the 13 existence of a lower-energy transition structure with a transition vector orthogonal to 14 the original MERP. 15

See [73,74]. 16 17 bimolecular 18

See molecularity. 19 [3] 20

21 binding site 22 Specific region (or atom) in a molecular entity that is capable of entering into a 23 stabilizing interaction with another atomic or molecular entity. 24

Example: an active site in an enzyme that interacts with its substrate. 25 Note 1: Typical modes of interaction are by covalent bonding, hydrogen bonding, 26

coordination, and ion-pair formation, as well as by dipole-dipole interactions, dispersion 27 forces, hydrophobic interactions, and desolvation. 28

Note 2: Two binding sites in different molecular entities are said to be complementary 29 if their interaction is stabilizing. 30

[3] 31 32

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biradical 1 diradical 2

See [9]. 3 rev[3] 4

5 blue shift 6 Informal expression for hypsochromic shift, but this historical term is discouraged 7 because it applies only to visible transitions. 8

rev[3] 9 10 Bodenstein approximation 11

See steady state. 12 [3] 13

14 bond 15 Balance of attractive and repulsive forces between two atoms or groups of atoms, 16 resulting in sufficient net stabilization to lead to the formation of an aggregate 17 conveniently considered as an independent molecular entity. 18

Note: The term usually refers to the covalent bond. 19 See [75]. 20 See also agostic, coordination, hydrogen bond, multi-centre bond. 21 rev[3] 22

23 bond dissociation 24

See heterolysis, homolysis. 25 Note: In ordinary usage the term refers to homolysis. If not, it should be specified as 26

heterolytic. 27 [3] 28

29 bond-dissociation energy 30 De 31 (SI unit: kJ mol–1) 32 Energy required to break a given bond of some specific molecular entity by homolysis 33 from its potential-energy minimum. 34

Note: This is the quantity that appears in the Morse potential. 35 See also bond-dissociation enthalpy. 36 rev[3] 37

38 bond-dissociation enthalpy 39 DH0, DdissH0 40

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(SI unit: kJ mol–1) 1 Standard molar enthalpy required to break a given bond of some specific molecular 2 entity by homolysis. 3

Example: For CH4 → CH3· + H· the bond-dissociation enthalpy is symbolized as 4 DH0(CH3–H). 5

Note: Although DH0 is commonly used, DdissH0 is more consistent with the notation 6 for other thermodynamic quantities. 7

See also bond-dissociation energy, bond energy, heterolytic bond-dissociation 8 enthalpy. 9 10 bond energy 11 D0 12 (SI unit: kJ mol–1) 13 Enthalpy of bond dissociation at 0 K. 14

See bond-dissociation enthalpy. 15 rev[3] 16

17 bond enthalpy (mean bond enthalpy, mean bond energy) 18 Average value of the gas-phase bond-dissociation enthalpies (usually at a temperature 19 of 298 K) for all bonds of the same type within the same chemical species. 20

Example: For methane, the mean bond enthalpy is 415.9 kJ mol–1, one-fourth the 21 enthalpy of reaction for 22

CH4(g) → C(g) + 4 H(g) 23 Note: More commonly, tabulated mean bond energies (which are really enthalpies) 24

are values of bond enthalpies averaged over a number of selected chemical species 25 containing that type of bond, such as 414 kJ mol–1 for C–H bonds in a group of R3CH 26 (R = H, alkyl). 27

See [76]. 28 29 bond order 30 Theoretical index of the degree of bonding between two atoms, relative to that of a 31 normal single bond, i.e., the bond provided by one localized electron pair. 32

Example: In ethene the C–C bond order is 2, and the C–H bond order is 1. 33 Note 1: In valence-bond theory it is a weighted average of the bond orders between 34

the respective atoms in the various resonance forms. In molecular-orbital theory it is 35 calculated from the weights of the atomic orbitals in each of the occupied molecular 36 orbitals. For example, in valence-bond theory the bond order between adjacent carbon 37 atoms in benzene is 1.5; in Hückel molecular orbital theory it is 1.67. 38

Note 2: Bond order is often derived from the electron distribution. 39

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Note 3: The Pauling bond order n (as often used in the bond-energy-bond-order 1 model) is a simple function of change in bond length d, where the value of the coefficient 2 c is often 0.3 Å (for n > 1) or 0.6 Å (for n < 1). 3 4

n = exp[(d1 – dn)/c] 5 rev[3] 6

7 bond-energy-bond-order model (BEBO): 8 Empirical procedure for estimating activation energy, involving relationships among 9 bond length, bond-dissociation energy, and bond order. 10

See [13,77]. 11 12 bond-stretch isomers 13 Two (or more) molecules with the same spin multiplicity but with different lengths for 14 one or more bonds. 15

Note: This feature arises because the potential-energy surface, which describes how 16 the energy of the molecule depends on geometry, shows two (or more) minima that are 17 not merely symmetry-related. 18

See [78,79,80,81]. 19 20 borderline mechanism 21 Mechanism intermediate between two extremes, for example a nucleophilic substitution 22 intermediate between SN1 and SN2, or intermediate between electron transfer and SN2. 23

[3] 24 25 Born-Oppenheimer approximation 26 Representation of the complete wavefunction as a product of electronic and nuclear 27 parts, Y(r,R) = yel(r,R) ynuc(R), so that the two wavefunctions can be determined 28 separately by solving two different Schrödinger equations. 29

See [8]. 30 31 Bredt's rule 32 Prohibition of placing a double bond with one terminus at the bridgehead atom of a 33 polycyclic system unless the rings are large enough to accommodate the double bond 34 without excessive strain. 35

Example: Bicyclo[2.2.1]hept-1-ene (A), which is capable of existence only as a 36 transient species, although its higher homologues, bicyclo[3.3.1]non-1-ene (B) and 37 bicyclo[4.2.1]non-1(8)-ene (C), with double bond at the bridgeheads, have been 38 isolated. 39

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1 2 3 4

A B C 5 6

See [82,83,84,85]. 7 For limitations see [86]. 8 Note: For an alternative formulation, based on the instability of a trans double bond 9

in a small ring (fewer than 8 atoms), see [87]. 10 rev[3] 11

12 bridged carbocation 13 Carbocation (real or hypothetical) in which there are two (or more) carbon atoms that 14 could in alternative Lewis formulas be designated as carbenium centres but which is 15 instead represented by a structure in which a group (a hydrogen atom or a hydrocarbon 16 residue, possibly with substituents in non-involved positions) bridges these potential 17 carbenium centres. 18

Note: Electron-sufficient bridged carbocations are distinguished from electron-19 deficient bridged carbocations. Examples of the former, where the bridging uses p 20 electrons, are phenyl-bridged ions (for which the trivial name phenonium ion has been 21 used), such as A. These ions are straightforwardly classified as carbenium ions. The 22 latter type of ion necessarily involves three-centre bonding because the bridging uses 23 σ electrons. The hydrogen-bridged carbocation B contains a two-coordinate hydrogen 24 atom, whereas structures C, D, and E (the 2-norbornyl cation) contain five-coordinate 25 carbon atoms. 26 27

28 29

See [88]. 30 See also carbonium ion, multi-centre bond, neighbouring group participation. 31 For the definitive X-ray structure of the norbornyl cation see [89]. 32 rev[3] 33

34 35 36

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bridgehead (atom) 1 Atom that is part of two or more rings in a polycyclic molecule and that is separated 2 from another bridgehead atom by bridges all of which contain at least one other atom. 3

Example: C1 and C4 in bicyclo[2.2.1]heptane, but not C4a and C8a in 4 decahydronaphthalene. 5 6 bridging ligand 7 Ligand attached to two or more, usually metallic, central atoms. 8

See [29]. 9 [3] 10

11 Brønsted acid (Brönsted acid) 12 Molecular entity capable of donating a hydron (proton) to a base (i.e., a hydron donor), 13 or the corresponding chemical species. 14

Examples: H2O, H3O+, CH3COOH, H2SO4, HSO4–, HCl, CH3OH, NH3. 15 See also conjugate acid-base pair. 16 [3] 17

18 Brønsted base (Brönsted base) 19 Molecular entity capable of accepting a hydron (proton) from a Brønsted acid (i.e., a 20 hydron acceptor), or the corresponding chemical species. 21

Examples: HO–, H2O, CH3CO2–, HSO4–, SO42–, Cl–, CH3O–, NH2–. 22 See also conjugate acid-base pair. 23 [3] 24

25 Brønsted relation (Brönsted relation) 26 Either of the equations 27 28 lg({kHA}/p) = C + a lg(q{KHA}/p) 29 30 lg({kA}/q) = C – b lg(q{KHA}/p) 31 32 wherea, b, and C are constants for a given reaction series (a and b are called Brønsted 33 exponents or Brønsted parameters). The arguments in the lg functions should be of 34 dimension 1. Thus, reduced rate coefficients should be used: {kA} = kA/[kA] and {kHA} = 35 kHA/[kHA] , which are the reduced catalytic coefficients of reactions whose rates depend 36 on the concentrations of acid HA or of its conjugate base A–, {KHA} = KHA/[KHA] is the 37 reduced acid dissociation constant of HA, p is the number of equivalent acidic protons 38 in HA, and q is the number of equivalent basic sites in A–. The chosen values of p and 39

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q should always be specified. (The charge designations of HA and A– are only 1 illustrative.). 2

Note1: The equations are often written without reduced variables, whereupon the 3 slope a or b, obtained from a graph or least-squares analysis, is correct because it is 4 the derivative of a logarithmic quantity, of dimension 1. 5

Note 2: The Brønsted relation is often termed the Brønsted catalysis law. Although 6 justifiable on historical grounds, use of this name is not recommended, since Brønsted 7 relations are known to apply to many uncatalysed and pseudo-catalysed reactions 8 (such as simple proton [hydron] transfer reactions). 9

Note 3: The term pseudo-Brønsted relation is sometimes used for reactions that 10 involve nucleophilic catalysis instead of acid-base catalysis. Various types of Brønsted 11 parameters have been proposed such as bnuc or blg for nucleophile or leaving group, 12 respectively. 13

See also linear free-energy relation (linear Gibbs energy relation). 14 rev[3] 15

16 Bunnett-Olsen equations 17 Relations between lg([SH+]/[S]) + Ho and Ho + lg{[H+]} for base S in aqueous acid 18 solutions, where Ho is Hammett's acidity function and Ho + lg{[H+]} represents the 19 activity function lg({gS} {gH+} / {gSH+}) for the nitroaniline reference bases to build Ho. and 20 where f is an empirical parameter that is determined by the slope of the linear 21 correlation of lg([SH+]/[S]) – lg{[H+]} vs. Ho + lg{[H+]}. 22 23 lg([SH+]/[S]) – lg{[H+]} = (f – 1)(Ho + lg{[H+]}) + pKSH+ 24 25 lg([SH+]/[S]) + Ho = f (Ho + lg{[H+]}) + pKSH+ 26 27 Arguments in the lg functions should be of dimension 1. Thus, concentrations should 28 be divided by the respective unit (unless they are eliminated as in the ratio of two 29 concentrations), i.e., the reduced quantity should be used, indicated by curly brackets. 30

Note 1: These equations avoid using (or defining) an acidity function for each family 31 of bases, including those for which such a definition is not possible. In many cases, f 32 (or f – 1) values for base families defining an acidity function are very similar. Broadly, 33 the value of f is related to the degree of solvation of SH+. 34

Note 2: These equations are obsolete, and the Cox-Yates equation, with the 35 equivalent parameter m* (= 1 – f), is now preferred. 36

See [21,90]. 37 See also Cox-Yates equation. 38 rev[3] 39

40

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b, bnuc, blg 1 Parameter in a Brønsted relation expressing the sensitivity of the rate of deprotonation 2 to basicity. 3

Note: bnuc and blg are used to correlate nucleophilic reactivity and leaving-group 4 ability, respectively. 5 6 b-elimination 7 1,2-elimination 8 Transformation of the general type 9 10 XABY → A=B + XY (or X + Y, or X+ + Y–) 11 12 where the central atoms A and B are commonly, but not necessarily, carbon. 13

See also elimination. 14 15 cage 16 Aggregate of molecules, generally solvent molecules in the condensed phase, that 17 surrounds fragments formed by thermal or photochemical dissociation. 18

Note: Because the cage hinders the separation of the fragments by diffusion, they 19 may preferentially react with one another ("cage effect") although not necessarily to re-20 form the precursor species. 21

Example: 22

23 See also geminate recombination. 24 [3] 25

26 cage compound 27 Polycyclic compound capable of encapsulating another compound 28

Example (adamantane, where the central cavity is large enough to encapsulate He, 29 Ne, or Na+): 30 31 32 33 34 35

Note: A compound whose cage is occupied is called an inclusion complex. 36 See [91,92]. 37 rev[3] 38

39 40

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canonical form 1 resonance form 2

rev[3] 3 4 captodative effect 5 Combined action of an electron-withdrawing ("captor") substituent and an electron-6 releasing ("dative") substituent, both attached to a radical centre, on the stability of a 7 carbon-centred radical. 8

Example: 9 10 11 12 13 14

See [93,94,95]. 15 rev[3] 16

17 carbanion 18 Generic name for an anion containing an even number of electrons and having an 19 unshared pair of electrons on a carbon atom which satisfies the octet rule and bears a 20 negative formal charge in its Lewis structure or in at least one of its resonance forms. 21

Examples: 22

23 24

See also radical ion. 25 See [40,96]. 26 rev[3] 27

28 carbene 29 Generic name for the species H2C: ("methylidene") and substitution derivatives thereof, 30 containing an electrically neutral bivalent carbon atom with two nonbonding electrons. 31

Note 1: The nonbonding electrons may have antiparallel spins (singlet state) or 32 parallel spins (triplet state). 33

Note 2: Use of the alternative name "methylene" as a generic term is not 34 recommended. 35

See [40]. 36 See also diradical. 37 [3] 38

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1 carbenium ion 2 Generic name for a carbocation whose electronic structure can be adequately 3 described by two-electron-two-centre bonds. 4

Note 1: The name implies a hydronated carbene or a substitution derivative thereof. 5 Note 2: The term is a replacement for the previously used term carbonium ion, which 6

now specifies a carbocation with penta- or higher-coordinate carbons. The names 7 provide a useful distinction between tricoordinate and pentacoordinate carbons. 8

Note 3: To avoid ambiguity, the name should be avoided as the root for the 9 nomenclature of carbocations. For example, the term "ethylcarbenium ion" might refer 10 to either CH3CH2+ (ethyl cation, ethylium) or CH3CH2CH2+ (propyl cation, propylium). 11

See [40,97]. 12 rev[3] 13

14 carbenoid 15 Carbene-like chemical species but with properties and reactivity differing from the free 16 carbene, arising from additional substituents bonded to the carbene carbon. 17

Example: R1R2C(Cl)M (M = metal) 18 rev[3] 19

20 carbocation 21 Positive ion containing an even number of electrons and with a significant portion of the 22 excess positive charge located on one or more carbon atoms. 23

Note 1: This is a general term embracing carbenium ions, all types of carbonium 24 ions, vinyl cations, etc. 25

Note 2: Carbocations may be named by adding the word "cation" to the name of the 26 corresponding radical [97,98]. 27

Note 3: Such names do not imply structure (e.g., whether three-coordinated or five-28 coordinated carbon atoms are present) [98]. 29

See also bridged carbocation, radical ion. 30 See [40]. 31 [3] 32

33 carbonium ion 34 (1) Carbocation that contains at least one carbon atom with a coordination number of 35 five or greater. 36 (2) Carbocation whose structure cannot adequately be described by only two-electron 37 two-centre bonds. 38

Example: methanium (CH5+). 39

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Note 1: In most of the earlier literature this term was used for all types of 1 carbocations, including those that are now defined as a (tricoordinate) carbenium ion. 2

Note 2: To avoid ambiguity, the term should be avoided as the root for the 3 nomenclature of carbocations. For example, the name "ethylcarbonium ion" might refer 4 to either CH3CH2+ (ethyl cation) or CH3CH2CH2+ (propyl cation). 5

See [98,99]. 6 rev[3] 7

8 carbyne 9 methylidyne 10 Generic name for the species HC·: and substitution derivatives thereof, such as 11 EtOCO–C·: (2-ethoxy-2-oxoethylidyne), containing an electrically neutral univalent 12 carbon atom with three non-bonding electrons. 13

Note: Use of the alternative name "methylidyne" as a generic term is not 14 recommended. 15

[3] 16 17 Catalán solvent parameters 18 Quantitative measure of solvent polarity, based on the solvent’s hydrogen-bond-donor 19 ability, hydrogen-bond-acceptor ability, polarizability, and dipolarity. 20

See [100,101]. 21 See also solvent parameter. 22

23 catalyst 24 Substance that increases the rate of a chemical reaction (owing to a change of 25 mechanism to one having a lower Gibbs energy of activation) without changing the 26 overall standard Gibbs energy change (or position of equilibrium). 27

Note 1: The catalyst is both a reactant and product of the reaction, so that there is 28 no net change in the amount of that substance. 29

Note 2: At the molecular level, the catalyst is used and regenerated during each set 30 of microscopic chemical events leading from a molecular entity of reactant to a 31 molecular entity of product. 32

Note 3: The requirement that there be no net change in the amount of catalyst is 33 sometimes relaxed, as in the base catalysis of the bromination of ketones, where base 34 is consumed, but this is properly called pseudo-catalysis. 35

Note 4: Catalysis can be classified as homogeneous, in which only one phase is 36 involved, and heterogeneous, in which the reaction occurs at or near an interface 37 between phases. 38

Note 5: Catalysis brought about by one of the products of a reaction is called 39 autocatalysis. 40

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Note 6: The terms catalyst and catalysis should not be used when the added 1 substance reduces the rate of reaction (see inhibition). 2

Note 7: The above definition is adequate for isothermal-isobaric reactions, but under 3 other experimental conditions the state function that is lowered by the catalyst is not the 4 Gibbs activation energy but the quantity corresponding to those conditions (e.g., the 5 Helmholtz energy under isothermal-isochoric conditions). 6

See [13]. 7 See also autocatalytic reaction, bifunctional catalysis, catalytic coefficient, electron-8

transfer catalysis, general acid catalysis, general base catalysis, intramolecular 9 catalysis, micellar catalysis, Michaelis-Menten kinetics, phase-transfer catalysis, 10 pseudo-catalysis, rate of reaction, specific catalysis. 11

rev[3] 12 13 catalytic antibody (abzyme) 14 monoclonal antibody with enzymatic activity 15

Note 1: A catalytic antibody acts by binding its antigen and catalyzing a chemical 16 reaction that converts the antigen into desired products. Despite the existence of natural 17 catalytic antibodies, most of them were specifically designed to catalyze desired 18 chemical reactions. 19

Note 2: Catalytic antibodies are produced through immunization against a transition-20 state analogue for the reaction of interest. The resulting antibodies bind strongly and 21 specifically the transition-state analogue, so that they become catalysts for the desired 22 reaction. 23

Note 3: The concept of catalytic antibodies and the strategy for obtaining them were 24 advanced by W. P. Jencks [102]. The first catalytic antibodies were finally produced in 25 1986 [103,104]. 26 27 catalytic coefficient 28 If the rate of reaction v is expressible in the form 29 30 v = (k0 + Ski[Ci]ni) [A]a [B]b... 31 32 where A, B, ... are reactants and Ci represents one of a set of catalysts, then the 33 proportionality factor ki is the catalytic coefficient of the particular catalyst Ci. 34

Note: Normally the partial order of reaction (ni) with respect to a catalyst will be unity, 35 so that ki is an (a + b+...+1)th-order rate coefficient. 36

[3] 37 38 39 40

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catalytic cycle 1 Sequence of reaction steps in the form of a loop. One step is binding of a reactant to 2 the active catalyst (sometimes formed from a precatalyst), and another step is the 3 release of product and regeneration of catalyst. 4

Example#!!A + B → C, catalyzed by X formed from precatalyst X'. 5

6 cation/p interaction 7 Noncovalent attractive force between a positive ion (metal cation, protonated Brønsted 8 base, etc.) and a p electron system. 9

See [105]. 10 11 chain reaction 12 Reaction in which one or more reactive reactive intermediates (frequently radicals) are 13 continuously regenerated through a repetitive cycle of elementary steps ("propagation 14 steps"). 15

Example: Chlorination of methane by a radical mechanism, where Cl· is continually 16 regenerated in the chain-propagation steps: 17

Cl· + CH4!→ HCl + H3C· 18 H3C· + Cl2!!→ CH3Cl + Cl· 19

Note: In chain polymerization reactions, reactive intermediates of the same types, 20 generated in successive steps or cycles of steps, differ in molecular mass, as in 21

RCH2C·HPh + H2C=CHPh → RCH2CHPhCH2C·HPh 22 See [106]. 23 See also chain transfer, initiation, termination. 24 [3] 25

26 chain transfer 27 Chemical reaction during a chain polymerization in which the active centre is transferred 28 from the growing macromolecule to another molecule or to another site on the same 29 molecule, often by abstraction of an atom by the radical end of the growing 30 macromolecule. 31

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Note 1: The growth of the polymer chain is thereby terminated but a new radical, 1 capable of chain propagation and polymerization, is simultaneously created. For the 2 example of alkene polymerization cited for a chain reaction, the reaction 3

RCH2C·HPh + CCl4!→!"$%2CHClPh + Cl3C· 4

represents a chain transfer, the radical Cl3C· inducing further polymerization: 5 Note 2: Chain transfer also occurs in other chain reactions, such as cationic or 6

anionic polymerization, in which case the abstraction is by the reactive cationic or 7 anionic end of the growing chain. 8

See [106]. 9 See also telomerization. 10 [3] 11

12 charge density 13

See electron density. 14 See [12]. 15 [3] 16

17 charge-transfer (CT) complex 18 Ground-state adduct that exhibits an electronic absorption corresponding to light-19 induced transfer of electronic charge from one region of the adduct to another. 20

See [9]. 21 rev[3] 22

23 chelation 24 Formation or presence of bonds (or other attractive interactions) between a single 25 central atom (or ion) and two or more separate binding sites within the same ligand. 26

Note 1: A molecular entity in which there is chelation (and the corresponding 27 chemical species) is called a chelate, while the species that binds to the central atom 28 is called a chelant. 29

Note 2: The terms bidentate, tridentate, ... multidentate are used to indicate the 30 number of potential binding sites of the ligand, at least two of which must be used by 31 the ligand in forming a chelate. For example, the bidentate ethylenediamine forms a 32 chelate with Cu+2 in which both nitrogen atoms of ethylenediamine are bonded to 33 copper. 34

Note 3: The use of the term is often restricted to metallic central atoms or ions. 35 Note 4: The phrase "separate binding sites" is intended to exclude cases such as 36

[PtCl3(CH2=CH2)]–, ferrocene, and (benzene)tricarbonylchromium, in which ethene, the 37 cyclopentadienyl group, and benzene, respectively, are considered to present single 38 binding sites to the respective metal atom. 39

See also ambident, cryptand. 40

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See also [29]. 1 [3] 2

3 cheletropic reaction 4 Cycloaddition across the terminal atoms of a π system with formation of two new s 5 bonds to a single atom of a monocentric reagent. There is formal loss of one p bond in 6 the substrate and an increase in coordination number of the relevant atom of the 7 reagent. 8

Example: addition of sulfur dioxide to butadiene: 9

10 Note: The reverse of this type of reaction is designated "cheletropic elimination". 11 See [107]. 12 [3] 13

14 chelotropic reaction 15 Alternative (and etymologically more correct) name for cheletropic reaction. 16

See [51]. 17 [3] 18

19 chemical flux, f 20 Unidirectional rate of reaction, applicable to the progress of component reaction steps 21 in a complex system or to the progress of reactions in a system at dynamic equilibrium 22 (in which there are no observable concentration changes with time), excluding the 23 reverse reaction and other reaction steps. 24

Note 1: Chemical flux is a derivative with respect to time, and has the dimensions of 25 amount of substance per volume transformed per time. 26

For example, for the mechanism 27 A + B ⇄ C 28 C + D → E 29 f1 is the chemical flux due to forward reaction step 1, or the rate of formation of C or 30

the rate of loss of A or B due to that step, and similarly forf–1 and f2. 31 Note 2: The sum of all the chemical fluxes leading to C is designated the "total 32

chemical flux into C" (symbol SfC), and the sum of all the chemical fluxes leading to 33 destruction of C is designated the "total chemical flux out of C" (symbol Sf–C), and 34 similarly for A and B. It then follows for this example that SfC = f1 and Sf–C =f–1 + f2, 35

Note 3: The net rate of appearance of C is then given by 36

SO2CH2=CH-CH=CH2 + SO2

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1 d[C]/dt = SfC – Sf–C 2

3 Note 4: In this system f1 (or SfC) can be regarded as the hypothetical rate of 4

formation of C due to the single (unidirectional) reaction 1 proceeding in the assumed 5 absence of all other reactions, and Sf–C can be regarded as the hypothetical rate of 6 destruction of C due to the two (unidirectional) reactions –1 and 2. 7

Note 5: Even when there is no net reaction, chemical flux can often be measured by 8 NMR methods. 9

See [1]. 10 See also order of reaction, rate-limiting step, steady state. 11 rev[3] 12

13 chemical reaction 14 Process that results in the interconversion of chemical species. 15

Note 1: This definition includes experimentally observable interconversions of 16 conformers and degenerate rearrangements. 17

Note 2: Chemical reactions may be elementary reactions or stepwise reactions. 18 Note 3: Detectable chemical reactions normally involve sets of molecular entities, as 19

indicated by this definition, but it is often conceptually convenient to use the term also 20 for changes involving single molecular entities (i.e., "microscopic chemical events"), 21 whose reactions can now be observed experimentally. 22

See also identity reaction. 23 rev[3] 24

25 chemical relaxation 26 Passage of a perturbed system toward or into chemical equilibrium. 27

Note 1: A chemical reaction at equilibrium can be disturbed from equilibrium by a 28 sudden change of some external parameter such as temperature, pressure, or electric-29 field strength. 30

Note 2: In many cases, and in particular when the displacement from equilibrium is 31 slight, the progress of the system towards equilibrium can be expressed as a first-order 32 process 33 34

c(t) – (ceq)2 = [(ceq)1 – (ceq)2] exp(–t/t) 35 36 where (ceq)1 and (ceq)2 are the equilibrium amount concentrations of one of the chemical 37 species before and after the change in the external parameter, and c(t) is its amount 38 concentration at time t. The time parametert, called relaxation time, is related to the 39

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rate coefficients of the chemical reaction involved. Such measurements are commonly 1 used to follow the kinetics of very fast reactions. 2

Note 3: Relaxation, or the passage toward equilibrium, is more general than chemical 3 relaxation, and includes relaxation of nuclear spins. 4

See [108,109]. 5 See relaxation. 6 rev[3] 7

8 chemical shift (NMR) 9 d 10 Variation of the resonance frequency of a nucleus in nuclear magnetic resonance 11 (NMR) spectrometry as a consequence of its environment. 12

Note 1: The chemical shift of a nucleus X, dX, expressed as its frequency, nX, relative 13 to that of a standard, nref, and defined as 14 15

dX = (nX – nref)/nref 16 17 For 1H and 13C NMR the reference signal is usually that of tetramethylsilane (SiMe4). 18

Note 2: Chemical shift is usually reported in "parts per million" or ppm, where the 19 numerator has unit Hz, and the denominator has unit MHz, like the spectrometer's 20 operating frequency. 21

Note 3: For historical reasons that predate Fourier-transform NMR, if a resonance 22 signal occurs at higher frequency than a reference signal, it is said to be downfield, and 23 if resonance occurs at lower frequency, the signal is upfield. Resonances downfield 24 from SiMe4 have positive d -values, and resonances upfield from SiMe4 have negative 25 d -values. These terms have been superseded, and deshielded and shielded are 26 preferred for downfield and upfield, respectively. 27

See [110]. 28 See also shielding. 29 rev[3] 30

31 chemical species 32 Ensemble of chemically identical molecular entities that can explore the same set of 33 molecular energy levels on the time scale of an experiment. The term is applied equally 34 to a set of chemically identical atomic or molecular structural units in a solid array. 35

Note 1: For example, conformational isomers may be interconverted sufficiently 36 slowly to be detectable by separate NMR spectra and hence to be considered to be 37 separate chemical species on a time scale governed by the radiofrequency of the 38 spectrometer used. On the other hand, in a slow chemical reaction the same mixture of 39 conformers may behave as a single chemical species, i.e., there is virtually complete 40

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equilibrium population of the total set of molecular energy levels belonging to the two 1 conformers. 2

Note 2: Except where the context requires otherwise, the term is taken to refer to a 3 set of molecular entities containing isotopes in their natural abundance. 4

Note 3: The definition given is intended to embrace not only cases such as graphite 5 and sodium chloride but also a surface oxide, where the basic structural units may not 6 be capable of isolated existence. 7

Note 4: In common chemical usage, and in this Glossary, generic and specific 8 chemical names (such as radical or hydroxide ion) or chemical formulae refer either to 9 a chemical species or to a molecular entity. 10

[3] 11 12 chemically induced dynamic nuclear polarization (CIDNP) 13 Non-Boltzmann nuclear spin-state distribution produced in thermal or photochemical 14 reactions, usually from colligation and diffusion or disproportionation of radical pairs, 15 and detected by NMR spectroscopy as enhanced absorption or emission signals. 16

See [111,112]. 17 rev[3] 18

19 chemiexcitation 20 Generation, by a chemical reaction, of an electronically excited molecular entity from 21 reactants in their ground electronic states. 22

See [9]. 23 [3] 24

25 chemoselectivity 26 Feature of a chemical reagent that reacts preferentially with one of two or more different 27 functional groups. 28

Note 1: A reagent has a high chemoselectivity if reaction occurs with only a limited 29 number of different functional groups. For example, sodium tetrahydridoborate (NaBH4) 30 is a more chemoselective reducing agent than is lithium tetrahydridoaluminate (LiAlH4). 31 The concept has not been defined in more quantitative terms. 32

Note 2: The term is also applied to reacting molecules or intermediates that exhibit 33 selectivity towards chemically different reagents. 34

Note 3: Usage of the term chemospecificity for 100 % chemoselectivity is 35 discouraged. 36

See [113]. 37 See also regioselectivity, stereoselectivity, stereospecificity. 38 [3] 39

40

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chemospecificity 1 obsolete 2

See chemoselectivity. 3 [3] 4

5 chirality 6 Property of a structure that is not superimposable on its mirror image. 7

See [11,46]. 8 [3] 9

10 chirality centre 11 chiral centre (superseded) 12 Atom with attached groups such that the arrangement is not superimposable on its 13 mirror image. 14

Note: Often this is a tetrahedral atom with four different groups attached, such as 15 CHBrClF, or C2 of CH3CHBrCH2CH3, or the sulfur of CH3S(=O)Ph, where the lone pair 16 is considered as a fourth group. 17

See [11]. 18 See also stereogenic centre. 19 [3] 20

21 chiral feature 22 Structural characteristic rendering a molecule chiral. 23

Examples: four different substituents on a carbon atom (chirality centre), 24 conformational helix, chiral axis (as in allenes XCH=C=CHX). 25 26 chiral recognition 27 Attraction between molecules through noncovalent interactions that exhibit 28 complementarity only between partners with specific chirality. 29

See also molecular recognition. 30 31 chromophore 32 Part (atom or group of atoms) of a molecular entity in which the electronic transition 33 responsible for a given spectral band is approximately localized. 34

Note 1: The term arose originally to refer to the groupings that are responsible for a 35 dye's colour. 36

Note 2: The electronic transition can often be assigned as involving n, p, p*, s, and/or 37 s* orbitals whose energy difference falls within the range of the visible or UV spectrum. 38

Note 3: The term has been extended to vibrational transitions in the infrared. 39 See [9,114,115]. 40

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[3] 1 2 CIDNP 3 Acronym for chemically induced dynamic nuclear polarization. 4

[3] 5 6 cine-substitution 7 Substitution reaction (generally aromatic) in which the entering group takes up a 8 position adjacent to that occupied by the leaving group. 9

Example: 10 11

12 13

See also tele-substitution. 14 [3] 15

16 classical carbocation 17 Carbocation whose electronic structure can be adequately described by two-electron-18 two-centre bonds, i.e., synonymous with carbenium ion. 19 20 clathrate 21

See host, inclusion compound. 22 [3] 23

24 coarctate 25 Feature of a concerted transformation in which the primary changes in bonding occur 26 within a cyclic array of atoms but in which two nonbonding atomic orbitals on an atom 27 interchange roles with two bonding orbitals. 28

Example: epoxidation with dimethydioxirane 29 30

OCH3

BrOCH3

NH2

+ KNH2 NH3

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1 Note: Because the atomic orbitals that interchange roles are orthogonal, such a 2

reaction does not proceed through a fully conjugated transition state and is thus not a 3 pericyclic reaction. It is therefore not governed by the rules that express orbital 4 symmetry restrictions applicable to pericyclic reactions. 5

See [116]. 6 See also pseudopericyclic. 7

8 colligation 9 Formation of a covalent bond by combination or recombination of two radicals. 10

Note: This is the reverse of unimolecular homolysis. 11 Example: 12

HO· + H3C· → CH3OH 13 [3] 14

15 collision complex 16 Ensemble formed by two reaction partners, where the distance between them is equal 17 to the sum of the van der Waals radii of neighbouring atoms. 18

See also encounter complex. 19 [3] 20

21 common-ion effect (on rates) 22 Reduction in the rate of certain reactions of a substrate RX in solution [by a path that 23 involves a pre-equilibrium with formation of R+ (or R–) ions as reaction intermediates] 24 caused by the addition to the reaction mixture of an electrolyte solute containing the 25 "common ion" X– (or X+). 26

Example: the rate of solvolysis of chlorodiphenylmethane in acetone-water is 27 reduced by the addition of salts of the common ion Cl–, which causes a decrease in the 28 steady-state concentration of the diphenylmethyl cation: 29

30 Ph2CHCl ⇄ Ph2CH+ + Cl– (free ions, not ion pairs) 31 Ph2CH+ + 2 OH2 → Ph2CHOH + H3O+ 32 33

C C

OO

C C

OO

‡

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Note: This retardation due to a common ion should be distinguished from the 1 acceleration due to a salt effect of all ions. 2

rev[3] 3 4 compensation effect 5 Observation that a plot of T∆rS vs. ∆rH (frequently T∆‡S vs. D‡H) for a series of reactions 6 with a range of different substituents (or other unique variable such as solvent or 7 dissolved salt), are straight lines of approximately unit slope, so that, e.g., the terms 8 ∆‡H and T∆‡S partially compensate, and ∆‡G = ∆‡H – T∆‡S shows less variation than 9 ∆‡H or T∆‡S separately. 10

Note: Frequently such ∆‡S vs. ∆‡H correlations are statistical artifacts, arising if 11 entropy and enthalpy are extracted from the variation of an equilibrium constant or a 12 rate constant with temperature, so that the slope and intercept of the van't Hoff plots 13 are correlated. The problem is avoided if the equilibrium constant and the enthalpy are 14 measured independently, for example by spectrophotometry and calorimetry, 15 respectively. 16

See [117,118,119,120]. 17 See also isoequilibrium relationship, isokinetic relationship. 18 rev[3] 19

20 complex 21 Molecular entity formed by loose association involving two or more component 22 molecular entities (ionic or uncharged), or the corresponding chemical species. The 23 attraction between the components is often due to hydrogen-bonding or van der Waals 24 attraction and is normally weaker than a covalent bond. 25

Note 1: The term has also been used with a variety of meanings in different contexts: 26 it is therefore best avoided when a more explicit alternative is applicable, such as adduct 27 when the association is a consequence of bond formation. 28

Note 2: In inorganic chemistry the term "coordination entity" is recommended instead 29 of "complex" [29,121]. 30

See also activated complex, adduct, charge transfer complex, electron-donor-31 acceptor complex, encounter complex, inclusion complex, s-adduct, p-adduct, 32 transition state. 33

[3] 34 35 composite reaction 36 Chemical reaction for which the expression for the rate of disappearance of a reactant 37 (or rate of appearance of a product) involves rate constants of more than a single 38 elementary reaction. 39

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Examples: "opposing reactions" (where rate constants of two opposed chemical 1 reactions are involved), "parallel reactions" (for which the rate of disappearance of any 2 reactant is governed by the rate constants relating to several simultaneous reactions 3 that form different products from a single set of reactants), and stepwise reactions. 4

[3] 5 6 comproportionation 7 Any chemical reaction!of the type A' + A" → 2 A 8

Example: 9 Pb + PbO2 + 2 H2SO4 → 2 PbSO4 + 2 H2O ( in a lead battery) 10 11

Note: Other stoichiometries are possible, depending on the oxidation numbers of the 12 species. 13

Reverse of disproportionation. The term "symproportionation" is also used. 14 See [122]. 15 rev[3] 16

17 concerted 18 Feature of a process in which two or more primitive changes occur within the same 19 elementary reaction. Such changes will normally be "energetically coupled". 20

Note 1: The term "energetically coupled" means that the simultaneous progress of 21 the primitive changes involves a transition state of lower energy than that for their 22 successive occurrence. 23

Note 2: In a concerted process the primitive changes may be synchronous or 24 asynchronous. 25

See also bifunctional catalysis, potential-energy surface. 26 [3] 27

28 condensation 29 Reaction (usually stepwise) in which two or more reactants (or remote reactive sites 30 within the same molecular entity) yield a product with accompanying formation of water 31 or some other small molecule, e.g., ammonia, ethanol, acetic acid, hydrogen sulfide. 32

Note 1: The mechanism of many condensation reactions has been shown to 33 comprise consecutive addition and elimination reactions, as in the base-catalysed 34 formation of (E)-but-2-enal (crotonaldehyde) from acetaldehyde, via dehydration of 3-35 hydroxybutanal (aldol). The overall reaction in this example is known as the aldol 36 condensation. 37

Note 2: The term is sometimes also applied to cases where the formation of water 38 or other simple molecule does not occur, as in "benzoin condensation". 39

[3] 40

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1 condensed formula 2 Linear representation of the structure of a molecular entity in which bonds are omitted. 3

Example: methyl 3-methylbutyl ether (isoamyl methyl ether, 4 (CH3)2CHCH2CH2OCH3, sometimes condensed further to (CH3)2CH[CH2]2OCH3) 5

Note: This term is sometimes also called a line formula, because it can be written on 6 a single line, but the line formula explicitly shows all bonds. 7 8 configuration (electronic) 9 Distribution of the electrons of an atom or a molecular entity over a set of one-electron 10 wavefunctions called orbitals, according to the Pauli principle. 11

Note: From one configuration several states with different multiplicities may result. 12 For example, the ground electronic configuration of the oxygen molecule (O2) is 13 1sg2, 1su2, 2sg2, 2su2, 1pu4, 3sg2, 1pg2 14 resulting in 15 3Sg–, 1Dg, and 1Sg+ multiplets 16

See [8,9]. 17 [3] 18

19 configuration (molecular) 20 Arrangement in space of the atoms of a molecular entity that distinguishes it from any 21 other molecular entity having the same molecular formula and connectivity and that is 22 not due to conformational differences (rotation about single bonds). 23

See [11]. 24 [3] 25

26 conformations 27 Different spatial arrangements of a molecular entity that can be interconverted by 28 rotation about one or more formally single bonds. 29

Note 1: Different conformations are often not considered to be stereoisomeric, 30 because interconversion is rapid. 31

Note 2: Different or equivalent spatial arrangements of ligands about a central atom, 32 such as those interconverted by pyramidal inversion (of amines) or Berry 33 pseudorotation (as of PF5) and other "polytopal rearrangements", are sometimes 34 considered conformations, but they are properly described as configurations. 35

See [11]. 36 rev[3] 37

38 conformational isomers 39 conformers 40

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1 conformer 2 Conformation of a molecular entity that corresponds to a minimum on the potential-3 energy surface of that molecular entity. 4

Note: The distinction between conformers and isomers is the height of the barrier for 5 interconversion. Isomers are stable on macroscopic timescales because the barrier for 6 interconversion is high, whereas a conformer cannot persist on a macroscopic 7 timescale because the interconversion between conformations is achieved rapidly. 8

See [11]. 9 10 11 conjugate acid 12 Brønsted acid BH+ formed on protonation (hydronation) of the base B. 13

Note 1: B is called the conjugate base of the acid BH+. 14 Note 2: The conjugate acid always carries one unit of positive charge more than the 15

base, but the absolute charges of the species are immaterial to the definition. For 16 example: the Brønsted acid HCl and its conjugate base Cl– constitute a conjugate acid-17 base pair, and so do NH4+ and its conjugate base NH3. 18

rev[3] 19 20 conjugated system, conjugation 21 Molecular entity whose structure may be represented as a system of alternating multiple 22 and single (or s) bonds: e.g., 23

CH2=CH–CH=CH2 or CH2=CH–CΞN 24 Note: In such systems, conjugation is the interaction of one p-orbital with another p-25

orbital (or d-orbital) across an intervening s bond, including the analogous interaction 26 involving a p-orbital containing an unshared electron pair, e.g., 27

28 or the interaction across a double bond whose p system does not interact with the p 29 orbitals of the conjugated system, as in 30

CH2=C=C=CH2 31 See also cross-conjugation, delocalization, resonance, through-conjugation. 32 [3] 33

34 connectivity 35 Description of which atoms are bonded to which other atoms. 36

Note: Connectivity is often displayed in a line formula or other structure showing 37 which atoms are bonded to which other atoms, but with minimal or no indication of bond 38 multiplicity. 39

Example: The connectivity of propyne is specified by CH3CCH. 40

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rev[3] 1 2 conrotatory 3 Stereochemical feature of an electrocyclic reaction in which the substituents at the 4 interacting termini of the conjugated system rotate in the same sense (both clockwise 5 or both counterclockwise). 6

See also disrotatory. 7 rev[3] 8

9 conservation of orbital symmetry 10

An approach to understanding pericyclic reactions that focuses on a symmetry 11 element (e.g., a reflection plane) that is retained along a reaction pathway. If each of 12 the singly or doubly occupied orbitals of the reactant(s) is of the same symmetry as a 13 similarly occupied orbital of the product(s), that pathway is "allowed" by orbital 14 symmetry conservation. If instead a singly or doubly occupied orbital of the reactant(s) 15 is of the same symmetry as an unoccupied orbital of the product(s), and an unoccupied 16 orbital of the reactant(s) is of the same symmetry as a singly or doubly occupied orbital 17 of the product(s), that pathway is "forbidden" by orbital symmetry conservation. 18

Note 1: This principle permits the qualitative construction of correlation diagrams to 19 show how molecular orbitals transform and how their energies change during chemical 20 reactions. 21

Note 2: Considerations of orbital symmetry are frequently grossly simplified in that, 22 for example, the p and p* orbitals of a carbonyl group in an asymmetric molecule are 23 treated as having the same topology (pattern of local nodal planes) as those of a 24 symmetric molecule (e.g., CH2=O), despite the absence of formal symmetry elements. 25

See [107,123]. 26 See also orbital symmetry. 27

28 constitutional isomers 29 Species (or molecular entities) with the same atomic composition (molecular formula) 30 but with different connectivity. 31

Note: The term structural isomers is discouraged, because all isomers differ in 32 structure and because isomers may be constitutional, configurational, or 33 conformational. 34

See [11]. 35 36 contributing structure 37 resonance form 38

rev[3] 39 40

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coordinate covalent bond 1 dative bond 2 3 coordination 4 Formation of a covalent bond, the two shared electrons of which come from only one 5 of the two partners linked by the bond, as in the reaction of a Lewis acid and a Lewis 6 base to form a Lewis adduct; alternatively, the bonding formed in this way. 7

Note: In the former sense, it is the reverse of unimolecular heterolysis. 8 See also dative bond, p-adduct. 9 rev[3] 10

11 coordination number 12 Number of other atoms directly linked to a specified atom in a chemical species 13 regardless of the number of electrons in the bonds linking them [29, Rule IR-10.2.5]. 14 For example, the coordination number of carbon in methane or of phosphorus in 15 triphenylphosphane oxide (triphenylphosphine oxide) is four whereas the coordination 16 number of phosphor is five in phosphorus pentafluoride. 17

Note: The term is used in a different sense in the crystallographic description of ionic 18 crystals. 19

rev[3] 20 21 coronate 22

See crown ether. 23 rev[3] 24

25 correlation analysis 26 Use of empirical correlations relating one body of experimental data to another, with the 27 objective of finding quantitative relationships among the factors underlying the 28 phenomena involved. Correlation analysis in organic chemistry often uses linear Gibbs-29 energy relations (formerly linear free-energy relation, LFER) for rates or equilibria of 30 reactions, but the term also embraces similar analysis of physical (most commonly 31 spectroscopic) properties and of biological activity. 32

See [124,125,126,127,128]. 33 See also linear free-energy relation, quantitative structure-activity relationship 34

(QSAR). 35 [3] 36

37 coupling constant (spin-spin coupling constant) 38 J 39 (Unit: Hz) 40

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Quantitative measure of nuclear spin-spin coupling in nuclear magnetic resonance 1 spectroscopy. 2

Note: Spin-spin coupling constants have been correlated with atomic hybridization 3 and with molecular conformations. 4

See [129,130,131]. 5 rev[3] 6

7 covalent bond 8 Stabilizing interaction associated with the sharing of electron pairs between two atomic 9 centres of a molecular entity, leading to a characteristic internuclear distance. 10

See also agostic, coordination, hydrogen bond, multi-centre bond. 11 See [8]. 12 rev[3] 13

14 Cox–Yates equation 15 Generalization of the Bunnett-Olsen equation of the form 16 17 lg([SH+])/[S]) – lg{[H+]} = m*X + pKSH+ 18 19 where [H+] is the amount concentration of acid, X is the activity-coefficient ratio 20 lg({gs}{gH+}/{gSH+}) for an arbitrary reference base, pKSH+ is the thermodynamic 21 dissociation constant of SH+, and m* is an empirical parameter derived from linear 22 regression of the left-hand side vs. X. Arguments in the lg functions should be unitless. 23 Thus, the reduced quantities should be used: {[H+]} is [H+]/units. 24

Note: The function X is called excess acidity because it gives a measure of the 25 difference between the acidity of a solution and that of an ideal solution of the same 26 concentration. In practice X = – (Ho + lg{[H+]}) and m* = 1 – f, where Ho is the Hammett 27 acidity function and f is the slope in the Bunnett-Olsen equation. 28

See [132,133,134]. 29 See Bunnett-Olsen equation. 30 rev[3] 31

32 critical micellisation concentration (cmc) 33 critical micelle concentration 34 Relatively small range of concentrations separating the limit below which virtually no 35 micelles are detected and the limit above which virtually all additional surfactant 36 molecules form micelles. 37

Note 1: Many physical properties of surfactant solutions, such as conductivity or light 38 scattering, show an abrupt change at a particular concentration of the surfactant, which 39 can be taken as the cmc. 40

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Note 2: As values obtained using different properties are not quite identical, the 1 method by which the cmc is determined should be clearly stated. 2

See [135]. 3 See also inverted micelle. 4 rev[3] 5

6 cross-conjugation 7 Phenomenon of three conjugated groups, two pairs of which exhibit conjugation but 8 without through-conjugation of all three, as in 2-phenylallyl, benzoate anion, divinyl 9 ether, or m-xylylene [1,3-C6H4(CH2)2]. 10

See [64]. 11 rev[3] 12 13

crown ether 14 Molecular entity comprising a monocyclic ligand assembly that contains three or more 15 binding sites held together by covalent bonds and capable of binding a guest in a central 16 (or nearly central) position. The adducts formed are sometimes known as "coronates". 17 The best known members of this group are macrocyclic polyethers, such as 18-crown-18 6, containing several repeating units –CR2–CR2O– (where R is most commonly H). 19

20 See [136,137]. 21 See also host. 22 rev[3] 23

24 cryptand 25 Molecular entity comprising a cyclic or polycyclic assembly of binding sites that contains 26 three or more binding sites held together by covalent bonds, and which defines a 27 molecular cavity in such a way as to bind (and thus "hide" in the cavity) another 28 molecular entity, the guest (a cation, an anion or a neutral species), more strongly than 29 do the separate parts of the assembly (at the same total concentration of binding sites). 30

Note 1: The adduct thus formed is called a "cryptate". The term is usually restricted 31 to bicyclic or oligocyclic molecular entities. 32

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Example: 1

2 Note 2: Corresponding monocyclic ligand assemblies (crown ether) are sometimes 3

included in this group, if they can be considered to define a cavity in which a guest can 4 hide. The terms "podand" and "spherand" are used for certain specific ligand 5 assemblies. Coplanar cyclic polydentate ligands, such as porphyrins, are not normally 6 regarded as cryptands. 7

See [138]. 8 See also host. See also [139]. 9 [3] 10

11 curly arrows 12 Symbols for depicting the flow of electrons in a chemical reaction or to generate 13 additional resonance forms. 14

Note 1: The tail of the curly arrow shows where an electron pair originates, and the 15 head of the curly arrow shows where the electron pair goes. 16

Note 2: Single-headed curly arrows are used to depict the flow of unpaired electrons. 17 Examples: 18

19

20 21

See electron pushing. 22 23 Curtin-Hammett principle 24 Statement that in a chemical reaction that yields one product (X) from one isomer (A') 25 and a different product (Y) from another isomer (A") (and provided these two isomers 26 are rapidly interconvertible relative to the rate of product formation, whereas the 27 products do not undergo interconversion) the product composition is not directly related 28 to the relative concentrations of the isomers in the substrate; it is controlled only by the 29 difference in standard Gibbs energies (G‡A' – G‡A") of the respective transition states. 30

–:O: O:

R

O:–:O

RCH3 Br–I: H CH3Cl·

O O

O O

N

O

N

O

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Note 1: The product composition is given by [Y]/[X] = (kYk1)/(k–1kX) or KckY/kX, where 1 Kc is the equilibrium constant, [A"]/[A'], and where kY and kX are the respective rate 2 constants of their reactions; these parameters are usually unknown. 3

Note 2: The energy diagram below represents the transformation of rapidly 4 interconverting isomers A' and A" into products X and Y. 5 6

7 8

9 10

Note 3: A related concept is the Winstein-Holness equation for the overall rate of 11 product formation in a system of rapidly interconverting isomers A' and A". The overall 12 rate constant k is given by 13 14

k = x'kX + x"kY 15 16 where x' and x" are the respective mole fractions of the isomers at equilibrium [15]. 17 However, since x' and x" are simple functions of Kc, the apparent dependence of the 18 overall rate on the relative concentrations of A' and A" disappears. The overall rate 19 constant is the sum kX + KckY, which is a function of the same quantities that determine 20 the product ratio. 21

See [140]. See also [15,141,142]. 22 rev[3] 23

24 cybotactic region 25

Reaction coordinate

‡A'δΔ‡G ‡A"

Δ‡GYΔ‡GX

A'

A"

Gº

XY

‡

ΔrGº

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That part of a solution in the vicinity of a solute molecule in which the ordering of the 1 solvent molecules is modified by the presence of the solute molecule. The term solvent 2 "cosphere" of the solute has also been used. 3

See [143,144]. 4 [3] 5

6 cyclization 7 Formation of a ring compound from a chain by formation of a new bond. 8

See also annulation. 9 [3] 10

11 cycloaddition 12 Reaction in which two or more unsaturated molecules (or parts of the same molecule) 13 combine with the formation of a cyclic adduct in which there is a net reduction of the 14 bond multiplicity. 15

Note 1: The following two systems of notation have been used for the more detailed 16 specification of cycloadditions, of which the second, more recent system (described 17 under (2)) is preferred: 18

(1) An (i+j+...) cycloaddition is a reaction in which two or more molecules (or parts 19 of the same molecule), respectively, provide units of i, j,... linearly connected 20 atoms: these units become joined at their respective termini by new σ bonds so 21 as to form a cycle containing (i+j+...) atoms. In this notation, (a) a Diels-Alder 22 reaction is a (4+2) cycloaddition, (b) the initial reaction of ozone with an alkene is 23 a (3+2) cycloaddition, and (c) the reaction of norbornadiene below is a (2+2+2) 24 cycloaddition. (Parentheses are used to indicate the numbers of atoms, but 25 brackets are also used.) 26

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1 2

(2) The symbolism [i+j+...] for a cycloaddition identifies the numbers i, j,... of 3 electrons in the interacting units that participate in the transformation of reactants 4 to products. In this notation the reaction (a) and (b) of the preceding paragraph 5 would both be described as [2+4] cycloadditions, and (c) as a [2+2+2] 6 cycloaddition. The symbol a or s (a = antarafacial, s = suprafacial) is often added 7 (usually as a subscript after the number to designate the stereochemistry of 8 addition to each fragment. A subscript specifying the orbitals, viz., s, p (with their 9 usual significance) or n (for an orbital associated with a single atom), may be 10 added as a subscript before the number. Thus the normal Diels-Alder reaction is 11 a [4s+2s] or [p4s + p2s] cycloaddition, whilst the reaction 12

13

14 would be a [14a+2s] or [p14a + p2s] cycloaddition, leading to the stereoisomer shown, 15 with hydrogens anti. (Brackets are used to indicate the numbers of electrons, and 16 they are also used instead of parentheses to denote the numbers of atoms.) 17

H3CO2CC CCO2CH3+

O

O

O

+ O

O

O

OO+

-O OO

O

CH3O2C CO2CH3

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Note 2: Cycloadditions may be pericyclic reactions or (non-concerted) stepwise 1 reactions. The term "dipolar cycloaddition" is used for cycloadditions of 1,3-dipolar 2 compounds. 3

See [107,145,146]. 4 See also cheletropic reaction, ene reaction, pericyclic reaction. 5 rev[3] 6

7 cycloelimination 8 Reverse of cycloaddition. The term is preferred to the synonyms "cycloreversion", 9 "retro-addition", and "retrocycloaddition". 10

[3] 11 12 cycloreversion 13 obsolete 14

See cycloelimination. 15 [3] 16

17 dative bond 18 obsolescent 19 Coordination bond formed between two chemical species, one of which serves as a 20 donor and the other as an acceptor of the electron pair that is shared in the bond. 21

Examples: the N–B bond in H3N+–B–H3, the S–O bond in (CH3)2S+–O–. 22 Note 1: A distinctive feature of dative bonds is that their minimum-energy rupture in 23

the gas phase or in inert solvent follows the heterolytic bond-cleavage path. 24 Note 2: The term is obsolescent because the distinction between dative bonds and 25

ordinary covalent bonds is not useful, in that the precursors of the bond are irrelevant: 26 H3N+–B–H3 is the same molecule, with the same bonds, regardless of whether the 27 precursors are considered to have been H3N + BH3 or H3N·+ + –BH3. 28

See [8]. 29 See coordination. 30 rev[3] 31

32 degenerate chemical reaction 33

See identity reaction. 34 [3] 35

36 degenerate rearrangement 37 Chemical reaction in which the product is indistinguishable (in the absence of isotopic 38 labelling) from the reactant. 39

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Note 1: The term includes both "degenerate intramolecular rearrangements" and 1 reactions that involve intermolecular transfer of atoms or groups ("degenerate 2 intermolecular reactions"): both are degenerate isomerizations. 3

Note 2: The occurrence of degenerate rearrangements may be detectable by isotopic 4 labelling or by dynamic NMR techniques. For example: the [3,3]sigmatropic 5 rearrangement of hexa-1,5-diene (Cope rearrangement), 6

Note 3: Synonymous but less preferable terms are "automerization", "permutational 7 isomerism", "isodynamic transformation", "topomerization". 8

See [147]. 9 See also fluxional, molecular rearrangement, narcissistic reaction, valence isomer. 10 [3] 11

12 delocalization 13 Quantum-mechanical concept most usually applied in organic chemistry to describe the 14 redistribution of p electrons in a conjugated system, where each link has a fractional 15 double-bond character, or a non-integer bond order, rather than p electrons that are 16 localized in double or triple bonds. 17

Note 1: There is a corresponding "delocalization energy", identifiable with the 18 stabilization of the system relative to a hypothetical alternative in which formal 19

(localized) single and double bonds are present. Some degree of delocalization is 20 always present and can be estimated by quantum mechanical calculations. The effects 21 are particularly evident in aromatic systems and in symmetrical molecular entities in 22 which a lone pair of electrons or a vacant p-orbital is conjugated with a double bond 23 (e.g., carboxylate ions, nitro compounds, enamines, the allyl cation). 24

Note 2: Delocalization in such species may be represented by partial bonds or by 25 resonance (here symbolized by a two-headed arrow) between resonance forms. 26 27

28

C

R

O O

C

H

H2C CH2

C

R

O OC

R

O O

C

H

H2C CH2C

H

H2C CH2+

1/2–1/2–

+ +

– –:. . : : : : :

. . . . . .

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These examples also illustrate the concomitant delocalization of charge in ionic 1 conjugated systems. Analogously, delocalization of the spin of an unpaired electron 2 occurs in conjugated radicals. 3

Note 3: Delocalization is not limited to p electrons. Hyperconjugation is the 4 delocalization of electrons of s bonds. 5

[3] 6 7 dendrimer 8 Large molecule constructed from a central core with repetitive branching and multiple 9 functional groups at the periphery. 10

See [148]. 11 Example: (with 48 CH2OH at the periphery) 12

13

14 15

Note: The name comes from the Greek dendron (dendron), which translates to "tree". 16 Synonymous terms include arborols and cascade molecules. 17

See [148,149,150,151,152]. 18 19 deshielding 20

See shielding. 21 [3] 22

23 detachment 24 Reverse of attachment. 25

[3] 26 27 28 29

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detailed balancing 1 Principle that when equilibrium is reached in a reaction system (containing an arbitrary 2 number of components and reaction paths), as many atoms, in their respective 3 molecular entities, will pass forward in a given finite time interval as will pass backward 4 along each individual path. 5

Note 1: It then follows that the reaction path in the reverse direction must in every 6 detail be the reverse of the reaction path in the forward direction (provided that the 7 system is at equilibrium). 8

Note 2: The principle of detailed balancing is a consequence for macroscopic 9 systems of the principle of microscopic reversibility. 10

[3] 11 12 diamagnetism 13 Property of substances having a negative magnetic susceptibility (χ), whereby they are 14 repelled out of a magnetic field. 15

See also paramagnetism. 16 [3] 17

18 diastereoisomerism 19 Stereoisomerism other than enantiomerism. 20

See diastereoisomers [11]. 21 [3] 22

23 diastereomeric excess (diastereoisomeric excess) 24 x1 – x2, where x1 and x2 (with x1 + x2 = 1) are the mole fractions of two diastereoisomers 25 in a mixture, or the fractional yields of two diastereoisomers formed in a reaction. 26

Note: Frequently this term is abbreviated to d.e. 27 See stereoselectivity, diastereoisomers. 28 See [11]. 29

30 diastereomeric ratio 31 x1/x2, where x1 and x2 are the mole fractions of two diastereoisomers in a mixture formed 32 in a reaction. 33

Note: Frequently this term is abbreviated to d.r. 34 See stereoselectivity, diastereoisomers. 35 See [11]. 36

37 diastereoisomers (diastereomers) 38 Stereoisomers not related as mirror images of each other. 39

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Note: Diastereoisomers are characterized by differences in physical properties, and 1 by differences in chemical behaviour toward chiral as well as achiral reagents. 2

See [11]. 3 4 diastereoselectivity 5 Preferential formation in a chemical reaction of one diastereoisomer over another. 6

Note: This can be expressed quantitatively by the diastereoisomeric excess or by the 7 diastereomeric ratio, which is preferable because it is more closely related to a Gibbs-8 energy difference. 9

See [11]. 10 See selectivity. 11

12 dielectric constant 13 obsolete 14

See [12]. 15 See permittivity (relative). 16 rev[3] 17

18 dienophile 19 Ene or yne component of a Diels-Alder reaction, including compounds with hetero-20 double bonds and hetero-triple bonds. 21

See cycloaddition. 22 rev[3] 23

24 diffusion-controlled rate 25 See encounter-controlled rate, microscopic diffusion control. Contrast mixing control. 26

[3] 27 28 dimerization 29 Transformation of a molecular entity A to give a molecular entity A2. 30

Examples: 31 2 CH3· → CH3CH3 32 2 CH3COCH3 → (CH3)2C(OH)CH2COCH3 33 2 RCOOH → (RCOOH)2 34

See also association. 35 [3] 36

37 Dimroth-Reichardt ET parameter 38

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Quantitative measure of solvent polarity, based on the wavelength, lmax, of the longest-1 wavelength intramolecular charge-transfer absorption band of the solvatochromic 2 betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridin-1-ium-1-yl)phenolate. 3 4

5 See [17,153,154,155]. 6 See solvent parameter. 7 rev[3] 8

9 dipolar aprotic solvent 10

See dipolar non-HBD solvent. 11 rev[3] 12

13 dipolar non-HBD solvent (Non-hydrogen-bond donating solvent) 14 Solvent with a comparatively high relative permittivity ("dielectric constant"), greater 15 than ca. 15, and composed of molecules that have a sizable permanent dipole moment 16 and that, although it may contain hydrogen atoms, cannot donate suitably labile 17 hydrogen atoms to form strong solvent-solute hydrogen bonds. 18

Examples: dimethyl sulfoxide, acetonitrile, acetone, as contrasted with methanol and 19 N-methylformamide. 20

Note 1: The term “dipolar” refers to solvents whose molecules have a permanent 21 dipole moment, in contrast to solvents whose molecules have no permanent dipole 22 moment and should be termed “apolar” or "nonpolar". 23

Note 2: Non-HBD solvents are often called aprotic, but this term is misleading 24 because a proton can be removed by a sufficiently strong base. The aprotic nature of 25 a solvent molecule means that its hydrogens are in only covalent C–H bonds and not 26 in polar O–Hd+ or N–Hd+ bonds that can serve as hydrogen-bond donors. Use of 27 “aprotic” is therefore discouraged, unless the context makes the term unambiguous. 28

Note 3: It is recommended to classify solvents according to their capability to donate 29 or not donate, as well as to accept or not accept, hydrogen bonds to or from the solute, 30 as follows: 31

Hydrogen-bond donating solvents (short: HBD solvents), formerly protic solvents 32 Non-hydrogen-bond donating solvents (short: non-HBD solvents), formerly aprotic 33

solvents 34 Hydrogen-bond accepting solvents (short: HBA solvents) 35

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Non-hydrogen-bond accepting solvents (short: non-HBA solvents) 1 See [156,157]. 2

3 dipole-dipole excitation transfer 4

Förster resonance-energy transfer (FRET) 5 See [9]. 6

7 dipole-dipole interaction 8 Intermolecular or intramolecular interaction between molecules or groups having a 9 permanent electric dipole moment. The strength of the interaction depends on the 10 distance and relative orientation of the dipoles. 11

Note: A dipole/dipole interaction is a simplification of the electrostatic interactions 12 between molecules that originate from asymmetries in the electron densities. Such 13 interactions can be described more correctly by the use of higher-order multipole 14 moments. 15

See also van der Waals forces. 16 rev[3] 17

18 dipole-induced dipole forces 19

See van der Waals forces. 20 [3] 21

22 diradical 23 biradical 24 Even-electron molecular entity with two (possibly delocalized) radical centres that act 25 nearly independently of each other, e.g., 26

27 Note 1: Species in which the two radical centres interact significantly are often 28

referred to as "diradicaloids". If the two radical centres are located on the same atom, 29 the species is more properly referred to by its generic name: carbene, nitrene, etc. 30

Note 2: The lowest-energy triplet state of a diradical lies below or at most only a little 31 above its lowest singlet state (usually judged relative to kBT, the product of the 32 Boltzmann constant kB and the absolute temperature T). If the two radical centres 33 interact significantly, the singlet state may be more stable. The states of those diradicals 34 whose radical centres interact particularly weakly are most easily understood in terms 35 of a pair of local doublets. 36

CH2CH2

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Note 3: Theoretical descriptions of low-energy states of diradicals display two 1 unsaturated valences: the dominant valence-bond structures have two dots, the low-2 energy molecular orbital configurations have only two electrons in two approximately 3 nonbonding molecular orbitals, and two of the natural orbitals have occupancies close 4 to one. 5

See [9,158,159,160,161,162,163]. 6 See also carbene, nitrene. 7 [3] 8

9 dispersion forces 10

See London forces, van der Waals forces. 11 [3] 12

13 disproportionation 14 Any chemical reaction!of the type A + A ⇋ A' + A", where A, A' and A" are different 15 chemical species. 16

Examples: 17 2 R2COH· (ketyl radical) → R2C=O + R2CHOH 18 Cannizzaro reaction: 2 ArCH=O → ArCH2OH + ArCOOH 19

20 Note 1: The reverse of disproportionation is called comproportionation. 21 Note 2: A special case of disproportionation (or "dismutation") is "radical 22

disproportionation", exemplified by 23 24

·CH2CH3 + ·CH2CH3 → CH2=CH2 + CH3CH3 25 26

Note 3: A somewhat more restricted usage of the term prevails in inorganic 27 chemistry, where A, A' and A"!are of different oxidation states. 28

[3] 29 30 disrotatory 31 Stereochemical feature of an electrocyclic reaction in which the substituents at the 32 interacting termini of the conjugated system rotate in opposite senses (one clockwise 33 and the other counterclockwise). 34

See also conrotatory. 35 rev[3] 36

37 dissociation 38 Separation of a molecular entity into two or more molecular entities (or any similar 39 separation within a polyatomic molecular entity). 40

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Example: 1 NH4+(aq) → H3O+ + NH3 or CH3COOH(aq) → H3O+ + CH3CO2– 2

Note 1: Although the separation of the constituents of an ion pair into free ions is a 3 dissociation, the ionization that produces the ion pair is not a dissociation, because the 4 ion pair is a single molecular entity. 5

Note 2: The reverse of dissociation is association. 6 rev[3] 7

8 distonic ion 9 Radical ion in which charge and radical sites are separated. 10

Example: •CH2CH2OCH2+ 11 See [164,165]. 12 rev[3] 13

14 distortion interaction model (Activation Strain Model) 15 Method for analyzing activation energy as the sum of the energies to distort the 16 reactants into the geometries they have in transition states plus the energy of interaction 17 between the two distorted reactants. 18

See [166]. 19 20 distribution ratio 21 partition ratio 22 Ratio of concentrations of a solute in a mixture of two immiscible phases at equilibrium. 23

See also Hansch constant. 24 See [167]. 25

26 di-p-methane rearrangement 27 Photochemical reaction of a molecular entity with two p-systems separated by a 28 saturated carbon, to form an ene-substituted cyclopropane. 29

Pattern: 30

31 See [9,168]. 32

33 donicity (also called donor number, DN, which is a misnomer) 34 Quantitative experimental measure of the Lewis basicity of a molecule B, expressed as 35 the negative enthalpy of the formation of the 1:1 complex B·SbCl5. 36

Note: donicity is often used as a measure of a solvent’s basicity. 37

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See also acceptor parameter, Lewis basicity. 1 See [63,169,170]. 2

3 downfield 4 superseded but still widely used in NMR to mean deshielded. 5

See chemical shift, shielding. 6 rev[3] 7

8 driving force 9 (1) Negative of the Gibbs energy change (∆rG0) on going from the reactants to the 10 products of a chemical reaction under standard conditions. Also called affinity. 11 (2) Qualitative term that relates the favorable thermodynamics of a reaction to a specific 12 feature of molecular structure, such as the conversion of weaker bonds into stronger 13 (CH3–H + Br–Br!→ CH3–Br + H–Br), neutralization of an acid (Claisen condensation of 14 2 CH3COOEt + EtO– to CH3COCH–COOEt + EtOH), or increase of entropy 15 (cycloelimination of cyclohexene to butadiene + ethylene). 16

Note 1: This term is a misnomer, because favorable thermodynamics is due to 17 energy, not force. 18

Note 2: This term has also been used in connection with photoinduced electron 19 transfer reactions, to indicate the negative of the estimated standard Gibbs energy 20 change for the outer sphere electron transfer (∆ETG0) [9]. 21

rev[3] 22 23 dual substituent-parameter equation 24 Any equation that expresses substituent effects in terms of two parameters. 25

Note: In practice the term is used specifically for an equation for modeling the effects 26 of meta- and para-substituents X on chemical reactivity, spectroscopic properties, etc. 27 of a probe site in benzene or other aromatic system. 28

PX = rI sI + rR sR 29 where PX is the magnitude of the property for substituent X, expressed relative to the 30 property for X = H; sI and sR are inductive (or polar) and resonance substituent 31 constants, respectively, there being various scales for sR; rI and rR are the 32 corresponding regression coefficients. 33

See [171,172,173]. 34 See also extended Hammett equation, Yukawa-Tsuno equation. 35 [3] 36

37 dynamic NMR 38 NMR spectroscopy of samples undergoing chemical reactions. 39

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Note: Customarily this does not apply to samples where the composition of the 1 sample, and thus its NMR spectrum, changes with time, but rather to samples at 2 equilibrium, without any net reaction. The occurrence of chemical reactions is 3 manifested by features of the NMR line-shape or by magnetization transfer. 4

See chemical flux. 5 6 dyotropic rearrangement 7 Process in which two s bonds simultaneously migrate intramolecularly. 8

Example 9

10 See [174,175,176]. 11 rev[3] 12

13 educt 14 deprecated: usage strongly discouraged 15 starting material, reactant 16

Note: This term should be avoided and replaced by reactant, because it means 17 "something that comes out", not "something that goes in". 18

rev[3] 19 20 effective charge, Zeff 21 Net positive charge experienced by an electron in a polyelectronic atom, which is less 22 than the full nuclear charge because of shielding by the other electrons. 23

rev[3] 24 25 effective molarity (effective concentration) 26 Ratio of the first-order rate constant or equilibrium constant of an intramolecular 27 reaction involving two functional groups within the same molecular entity to the second-28 order rate constant or equilibrium constant of an analogous intermolecular elementary 29 reaction. 30

Note: This ratio has unit of concentration, mol dm–3 or mol L–1, sometimes denoted 31 by M. 32

See [177]. 33 See also intramolecular catalysis. 34 [3] 35

36 37

O

CH3CH2 R

OO

R

OCH3MgBr2

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eighteen-electron rule 1 Electron-counting rule that the number of nonbonding electrons at a metal plus the 2 number of electrons in the metal-ligand bonds should be 18. 3

Note: The 18-electron rule in transition-metal chemistry is an analogue of the Lewis 4 octet rule. 5

[3] 6 7 electrocyclic reaction (electrocyclization) 8 Molecular rearrangement that involves the formation of a s bond between the termini 9 of a fully conjugated linear p-electron system (or a linear fragment of a p-electron 10 system) and a decrease by one in the number of π bonds, or the reverse of that process. 11

Examples: 12

13 Note: The stereochemistry of such a process is termed "conrotatory" if the 14

substituents at the interacting termini of the conjugated system both rotate in the same 15 sense, as in 16

17 or "disrotatory" if one terminus rotates in a clockwise and the other in a 18 counterclockwise sense, as in 19

20 See also pericyclic reaction. 21 [3] 22

23 electrofuge 24 Leaving group that does not carry away the bonding electron pair. 25

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Examples: In the nitration of benzene by NO2+, H+ is the electrofuge, and in an SN1 1 reaction the carbocation is the electrofuge. 2

Note 1: Electrofugality characterizes the relative rates of atoms or groups to depart 3 without the bonding electron pair. Electrofugality depends on the nature of the reference 4 reaction and is not the reverse of electrophilicity [178]. 5

Note 2: For electrofuges in SN1 reactions see [179]. 6 See also electrophile, nucleofuge. 7 rev[3] 8

9 electromeric effect 10 obsolete 11

rev[3] 12 13 electron acceptor 14 Molecular entity to which an electron may be transferred. 15

Examples: 1,4-dinitrobenzene,1,1'-dimethyl-4,4'-bipyridinium dication, 16 benzophenone. 17

Note 1: A group that accepts electron density from another group is not called an 18 electron acceptor but an electron-withdrawing group. 19

Note 2: A Lewis acid is not called an electron acceptor but an electron-pair acceptor. 20 rev[3] 21

22 electron affinity 23 Energy released when an additional electron (without excess energy) attaches itself to 24 a molecular entity (often electrically neutral but not necessarily). 25

Note 1: Equivalent to the minimum energy required to detach an electron from a 26 singly charged negative ion. 27

Note 2: Measurement of electron affinities is possible only in the gas phase, but there 28 are indirect methods for evaluating them from solution data, such as polarographic half-29 wave potentials or charge-transfer spectra. 30

See [8,180,181]. 31 rev[3] 32

33 electron capture 34 Transfer of an electron to a molecular entity, resulting in a molecular entity of 35 (algebraically) increased negative charge. 36 37 electron-deficient bond 38 Bond between adjacent atoms that is formed by fewer than two electrons. 39

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Example: 1

2 Note: The B–H–B bonds are also called "two-electron three-centre bonds". 3 [3] 4

5 electron density 6 The electron density at a point with coordinates x,y,z in an atom or molecular entity is 7 the product of the probability P(x,y,z) (units: m-3) of finding an electron at that point with 8 the volume element dx dy dz (units: m3). 9

Note: For many purposes (e.g., X-ray scattering, forces on atoms) the system 10 behaves as if the electrons were spread out into a continuous distribution, which is a 11 manifestation of the wave-particle duality. 12

See also atomic charge, charge density. 13 rev[3] 14

15 electron donor 16 Molecular entity that can transfer an electron to another molecular entity, or to the 17 corresponding chemical species. 18

Note 1: After the electron transfer the two entities may separate or remain 19 associated. 20

Note 2: A group that donates electron density to another group is an electron-21 donating group, regardless of whether the donation is of s or p electrons. 22

Note 3: A Lewis base is not called an electron donor, but an electron-pair donor. 23 See also electron acceptor. 24

rev[3] 25 26 electron-donor-acceptor complex 27 obsolete 28 charge-transfer complex. 29

See also adduct, coordination. 30 31 electron-pair acceptor 32 Lewis acid. 33

[3] 34 35 36 37

HB

HBH H

H H

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electron-pair donor 1 Lewis base. 2

[3] 3 4 electron pushing 5 Method using curly arrows for showing the formal movement of an electron pair (from 6 a lone pair or a s or p bond) or of an unpaired electron, in order to generate additional 7 resonance forms or to denote a chemical reaction. 8

Note 1: The electron movement may be intramolecular or intermolecular. 9 Note 2: When a single electron is transferred, a single-headed curly arrow or “fish-10

hook” is used, but the electron movement in the opposite direction is redundant and 11 sometimes omitted. 12

Examples: 13

14 15 electron transfer 16 Transfer of an electron from one molecular entity to another, or between two localized 17 sites in the same molecular entity. 18

See also inner sphere (electron transfer), Marcus equation, outer sphere (electron 19 transfer). 20

[3] 21 22 electron-transfer catalysis 23 Process describing a sequence of reactions such as shown in equations (1)-(3), leading 24 from A to B, via species A·– and B·– that have an extra electron: 25

A + e– → A·– (1) 26 A·– → B·– (2) 27 B·– + A → B + A·– (3) 28

Note 1: An analogous sequence involving radical cations (A·+, B·+) also occurs. 29 Note 2: The most notable example of electron-transfer catalysis is the SRN1 (or 30

T+DN+AN) reaction of haloarenes with nucleophiles. 31 Note 3: The term has its origin in an analogy to acid-base catalysis, with the electron 32

instead of the proton. However, there is a difference between the two catalytic 33 mechanisms, since the electron is not a true catalyst, but rather behaves as the initiator 34 of a chain reaction. "Electron-transfer induced chain reaction" is a more appropriate 35 term for the mechanism described by equations (1)-(3). 36

–:O: O:

R

O:–:O

RCH3 Br–I: H CH3Cl·

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See [182,183]. 1 [3] 2

3 electronation 4 obsolete 5

See reduction. 6 rev[3] 7

8 electronegativity 9 Measure of the power of an atom to attract electrons to itself. 10

Note 1: The concept has been quantified by a number of authors. The first, due to 11 Pauling, is based on bond-dissociation energies, Ed (units: eV), and is anchored by 12 assigning the electronegativity of hydrogen as cr,H = 2.1. For atoms A and B 13 14 cr,A – cr,B = Ö{Ed(A–B)/eV – ½ [Ed(A–A) + Ed(B–B)]/eV} 15 16 where cr denotes the Pauling relative electronegativity; it is relative in the senses that 17 the values are of dimension 1 and that only electronegativity differences are defined on 18 this empirical scale. See [12] section 2.5. 19

Note 2: Alternatively, the electronegativity of an element, according to the Mulliken 20 scale, is the average of its atomic ionization energy and electron affinity. Other scales 21 have been developed by Allred and Rochow, by Sanderson, and by Allen. 22

Note 3: Absolute electronegativity is property derived from density functional theory 23 [8]. 24

See [75,184,185,186,187,188,189,190]. 25 rev[3] 26

27 electronic effects (of substituents) 28 Changes exerted by a substituent on a molecular property or molecular reactivity, often 29 distinguished as inductive (through-bond polarization), through-space electrostatics 30 (field effect), or resonance, but excluding steric. 31

Note 1: The obsolete terms mesomeric and electromeric are discouraged. 32 Note 2: Quantitative scales of substituent effects are available. 33 See [126,173,191]. 34 See also polar effect. 35 rev[3] 36

37 electrophile (n.), electrophilic (adj.) 38 Reagent that forms a bond to its reaction partner (the nucleophile) by accepting both 39 bonding electrons from that partner. 40

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Note 1: Electrophilic reagents are Lewis acids. 1 Note 2: "Electrophilic catalysis" is catalysis by Lewis acids. 2 Note 3: The term "electrophilic" is also used to designate the apparent polar 3

character of certain radicals, as inferred from their higher relative reactivities with 4 reaction sites of higher electron density. 5

See also electrophilicity. 6 [3] 7

8 electrophilic substitution 9 Heterolytic reaction in which the entering group adds to a nucleophile and in which the 10 leaving group, or electrofuge, relinquishes both electrons to its reaction partner, 11 whereupon it becomes another potential electrophile. 12

Example (azo coupling): 13 RN2+ (electrophile) + H2NC6H5 (nucleophile) → p-H2NC6H4N=NR + H+ (electrofuge) 14

Note: It is arbitrary to emphasize the electrophile and ignore the feature that this is 15 also a nucleophilic substitution, but the distinction depends on the electrophilic nature 16 of the reactant that is considered to react with the substrate. 17

See also substitution. 18 19 electrophilicity 20 Relative reactivity of an electrophile toward a common nucleophile. 21

Note: The concept is related to Lewis acidity. However, whereas Lewis acidity is 22 measured by relative equilibrium constants toward a common Lewis base, 23 electrophilicity is measured by relative rate constants for reactions of different 24 electrophilic reagents towards a common nucleophilic substrate. 25

See [192,193,194]. 26 See also nucleophilicity. 27 rev[3] 28

29 element effect 30 Ratio of the rate constants of two reactions that differ only in the identity of the element 31 in the leaving group. 32

Example: kBr/kCl for the reaction of N3– (azide) with CH3Br or CH3Cl. 33 rev[3] 34

35 elementary reaction 36

Reaction for which no reaction intermediates have been detected or need to be 37 postulated in order to describe the chemical reaction on a molecular scale. An 38 elementary reaction is assumed to occur in a single step and to pass through only one 39 transition state or none. 40

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See [13]. 1 See also composite reaction, stepwise reaction. 2 rev[3] 3

4 elimination 5 Reverse of an addition reaction. 6

Note 1: In an elimination two groups (called eliminands) are lost, most often from two 7 different centres (1,2-elimination (b-elimination) or 1,3-elimination, etc.) with 8 concomitant formation of an unsaturation (double bond, triple bond) in the molecule, or 9 formation of a ring. 10

Note 2: If the groups are lost from a single carbon or nitrogen centre (1,1-elimination, 11 a-elimination), the resulting product is a carbene or nitrene, respectively. 12

rev[3] 13 14 empirical formula 15 List of the elements in a chemical species, with integer subscripts indicating the simplest 16 possible ratios of all elements. 17

Note 1: In organic chemistry C and H are listed first, then the other elements in 18 alphabetical order. 19

Note 2: This differs from the molecular formula, in which the subscripts indicate how 20 many of each element is included and which is an integer multiple of the empirical 21 formula. For example, the empirical formula of glucose is CH2O while its molecular 22 formula is C6H12O6. 23

Note 3: The empirical formula is the information provided by combustion analysis, 24 which has been largely superseded by mass spectrometry, which provides the 25 molecular formula. 26 27 enantiomer 28 One of a pair of stereoisomeric molecular entities that are non-superimposable mirror 29 images of each other. 30

See [11]. 31 [3] 32

33 enantiomeric excess 34 Absolute value of the difference between the mole fractions of two enantiomers: 35 36

Dx(e.e.) = |x+ – x –| 37 38 where x+ + x– = 1. 39

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Note: Enantiomeric excess can be evaluated experimentally from the observed 1 specific optical rotatory power [a]obs, relative to [a]max, the (maximum) specific optical 2 rotatory power of a pure enantiomer: 3

4 Dx(e.e.) = | [a]obs/[a]max | 5

6 and also by chiral chromatography, NMR, and MS methods. 7

See [11]. 8 rev[3] 9

10 enantiomeric ratio 11 mole fraction of one enantiomer in a mixture divided by the mole fraction of the other. 12 13

r(e.r.) = x+/x– or x–/x+ 14 15 where x+ + x– = 1 16

See [11]. 17 rev[3] 18

19 enantioselectivity 20

See stereoselectivity. 21 [3] 22

23 encounter complex 24 Complex of molecular entities produced at an encounter-controlled rate, and which 25 occurs as an intermediate in a reaction. 26

Note 1: When the complex is formed from two molecular entities it is called an 27 "encounter pair". A distinction between encounter pairs and (larger) encounter 28 complexes may be relevant for mechanisms involving pre-association. 29

Note 2: The separation between the entities is small compared to the diameter of a 30 solvent molecule. 31

See also [9]. 32 rev[3] 33

34 encounter-controlled rate 35 Rate of reaction corresponding to the rate at which the reacting molecular entities 36 encounter each other. This is also known as the "diffusion-controlled rate", since rates 37 of encounter are themselves controlled by diffusion rates (which in turn depend on the 38 viscosity of the medium and the dimensions of the reacting molecular entities). 39

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Note: At 25 °C in most solvents, including water, a bimolecular reaction that proceeds 1 at an encounter-controlled rate has a second-order rate constant of 109 to 1010 dm3 2 mol–1 s–1. 3

See also microscopic diffusion control. 4 [3] 5

6 ene reaction 7 Addition of a compound with a double bond and an allylic hydrogen (the "ene") to a 8 compound with a multiple bond (the "enophile") with transfer of the allylic hydrogen and 9 a concomitant reorganization of the bonding. 10

Example, with propene as the ene and ethene as the enophile. 11

12 Note: The reverse is a "retro-ene" reaction. 13 [3] 14

15 energy of activation 16 Ea or EA 17 Arrhenius energy of activation 18 activation energy 19 (SI unit: kJ mol–1) 20 Operationally defined quantity expressing the dependence of a rate constant on 21 temperature according to 22 23

𝐸!(𝑇) =– 𝑅"#${!(#)[!] }

"''#( 24

25 as derived from the Arrhenius equation, k(T) = A exp(-EA/RT), where A is the pre-26 exponential factor and R the gas constant. As the argument of the ln function, k should 27 be divided by its units, i.e., by [k]. 28

Note 1: According to collision theory, the pre-exponential factor A is the frequency of 29 collisions with the correct orientation for reaction and Ea (or E0) is the threshold energy 30 that collisions must have for the reaction to occur. 31

H2CHC CH2

H CH2CH2

H2CHC

CH2

CH2

CH2H

+

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Note 2: The term Arrhenius activation energy is to be used only for the empirical 1 quantity as defined above. There are other empirical equations with different activation 2 energies, see [12]. 3

See [13]. 4 See also enthalpy of activation. 5 rev[3] 6

7 energy profile 8

See Gibbs energy diagram, potential-energy profile. 9 [3] 10

11 enforced concerted mechanism 12 Situation where a putative intermediate possesses a lifetime shorter than a bond 13 vibration, so that the steps become concerted. 14

See [195,196,197]. 15 rev[3] 16

17 enthalpy of activation (standard enthalpy of activation)!18

&‡Ho 19 (SI unit: kJ mol–1) 20 Standard enthalpy difference between the transition state and the ground state of the 21 reactants at the same temperature and pressure. It is related experimentally to the 22 temperature dependence of the rate coefficient k according to equation (1) for first-order 23 rate constants: 24 25

&‡Hº = –R"" #$%

!s#$%

& K⁄ &

"'$&(#

)

(1) 26

27 and to equation (2) for second order rate coefficients 28 29

&‡Hº = –R"! "#$

% &moldm3s-1/01 2⁄ %

!&41'#

(

(2) 30

31

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This quantity can be obtained, along with the entropy of activation!&‡Sº, from the slope 1

and intercept of the linear least-squares fit of rate coefficients k to the equation 2 3

ln))s-1+

, -⁄*!= ∆‡Sº/R – ∆‡Hº/RT + ln[ (kB/J K-1)/(h/J s) ] 4

for a first-order rate coefficient 5 and 6

ln+) /moldm3s-15⁄, -⁄

,'!&‡Sº/R!(!&‡Hº/R (1/T) + ln[ (kB/J K-1)/(h/J s) ] 7

8 for a second-order rate coefficient where kB is Boltzmann constant, h is Planck constant, 9 and kB/h = 2.08366… ´ 1010 K-1 s-1. 10

Note 1: An advantage of the least-squares fit is that it can also give error estimates 11 for ∆‡Hº and ∆‡Sº. 12

Note 2: It is also given by 13 14

∆‡Hº = RT2(∂ln k/[k]/∂T)P – RT = Ea – RT 15 16 ∆‡Hº = RT2(∂ln{k}//∂T)P – RT 17

18 where Ea is the energy of activation, provided that the rate coefficients for reactions 19 other than first-order are expressed in temperature-independent concentration units 20 (e.g., mol kg–1, measured at a fixed temperature and pressure). The argument in a 21 logarithmic function should be of dimension 1. Thus k is divided by its units, i.e., by [k], 22 as in the alternative form, in terms of reduced variables. 23

See also entropy of activation, Gibbs energy of activation. 24 See [12,13]. 25 rev[3] 26

27 entropy of activation, (standard entropy of activation) 28 ∆‡Sº 29 (SI unit: J mol–1 K–1) 30 Standard entropy difference between the transition state and the ground state of the 31 reactants, at the same temperature and pressure. It is related to the Gibbs energy of 32 activation and enthalpy of activation by the equations 33 34

∆‡Sº = (∆‡Hº –∆‡Gº)/T 35 36

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provided that rate coefficients for reactions other than first-order reactions are 1 expressed in temperature-independent concentration units (e.g., mol dm–3, measured 2 at fixed temperature and pressure). The numerical value of S depends on the standard 3 state (and therefore on the concentration units selected). 4

Note 1: It can also be obtained from the intercept of the linear least-squares fit of rate 5 coefficients k to the equation 6 7

ln(k/[k]/T[T]) = ∆‡Sº/R – ∆‡Hº/R (1/T) + ln(kB/h), 8 9

ln({k}/{T}) = ∆‡Sº/R – ∆‡Hº/R (1/T) + ln(kB/h), 10 11 where kB/h = 2.08366… ´ 1010 K-1 s-1. k should be divided by its units, i.e., by [k] = s-1 12 for first-order, and by [k] = (s-1 mol-1 dm3) for second-order rate coefficients and T should 13 be divided by its units [T] = K, as in the second equation, in terms of reduced variables.. 14

Note 2: The information represented by the entropy of activation may alternatively 15 be conveyed by the pre-exponential factor A, which reflects the fraction of collisions 16 with the correct orientation for reaction (see energy of activation). 17

See [12,13]. 18 rev[3] 19

20 epimer 21 Diastereoisomer that has the opposite configuration at only one of two or more 22 tetrahedral stereogenic centres in the respective molecular entity. 23

See [11]. 24 [3] 25

26 epimerization 27 Interconversion of epimers by reversal of the configuration at one of the stereogenic 28 centres. 29

See [11]. 30 [3] 31

32 equilibrium, chemical 33 Situation in which reversible processes (processes that may be made to proceed in 34 either the forward or reverse direction by the infinitesimal change of one variable) have 35 reached a point where the rates in both directions are identical, so that the amount of 36 each species no longer changes. 37

Note 1: In this situation the Gibbs energy, G, is a minimum. Also, the sum of the 38 chemical potentials of the reactants equals that of the products, so that 39

&rGº = –RT ln K 40

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where the thermodynamic equilibrium constant, K (of dimension 1), is the product of 1 product activities divided by the product of reactant activities. 2

Note 2: In dilute solutions the numerical values of the thermodynamic activities may 3 be approximated by the respective concentrations. 4

rev[3] 5 6 equilibrium control 7

See thermodynamic control. 8 [3] 9

10 ET-value 11 See Dimroth-Reichardt ET parameter, Z-value. 12

[3] 13 14 excess acidity 15 See Bunnett-Olsen equations, Cox-Yates equation. 16

[3] 17 18 excimer (“excited dimer”) 19 Complex formed by the interaction of an excited molecular entity with another identical 20 molecular entity in its ground state. 21

Note: The complex is not stable in the ground state. 22 See [9]. 23 See also exciplex. 24 [3] 25

26 exciplex 27 Electronically excited complex of definite stoichiometry that is non-bonding in the 28 ground state. 29

Note: An exciplex is a complex formed by the noncovalent interaction of an excited 30 molecular entity with the ground state of a different molecular entity, but an excimer, 31 formed from two identical components, is often also considered to be an exciplex. 32

See [9]. 33 See also excimer. 34 [3] 35

36 excited state 37 Condition of a system with energy higher than that of the ground state. This term is 38 most commonly used to characterize a molecular entity in one of its electronically 39

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excited states, but may also refer to vibrational and/or rotational excitation in the 1 electronic ground state. 2

See [9]. 3 [3] 4

5 EXSY (NMR exchange spectroscopy) 6 Two-dimensional NMR technique producing cross peaks corresponding to site-to site 7 chemical exchange. The cross-peak amplitudes carry information about exchange 8 rates. 9

See [198]. 10 11 extended Hammett equation 12 Multiparameter extension of the Hammett equation for the description of substituent 13 effects. 14

Note 1: The major extensions using two parameters (dual substituent-parameter 15 equations) were devised for the separation of inductive and steric effects (Taft equation) 16 or of inductive (or field) and resonance effects. 17

Note 2: Other parameters may be added (polarizability, hydrophobicity…) when 18 additional substituent effects are operative. 19

See [191]. 20 See also Yukawa-Tsuno equation. 21 [3] 22

23 external return (external ion-pair recombination) 24 Recombination of free ions formed in an SN1 reaction (as distinguished from ion-pair 25 collapse). 26

See ion-pair recombination. 27 rev[3] 28

29 extrusion 30 Transformation in which an atom or group Y connected to two other atoms or groups X 31 and Z is lost from a molecule, leading to a product in which X is bonded to Z, i.e., 32

X–Y–Z → X–Z + Y 33 Example 34

NN + N2

hν

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1 Note 1: When Y is a metal, this process is called a reductive elimination. 2 Note 2: The reverse of an extrusion is called an insertion. 3 See also cheletropic reaction. 4 [3] 5

6 field effect 7 Experimentally observable substituent effect (on reaction rates, etc.) of intramolecular 8 electrostatic interaction between the centre of interest and a monopole or dipole, by 9 direct electric-field action through space rather than through bonds. 10

Note 1: The magnitude of the field effect depends on the monopole charge or dipole 11 moment, on the orientation of the dipole, on the distance between the centre of interest 12 and the monopole or dipole, and on the effective dielectric constant that reflects how 13 the intervening bonds are polarized. 14

Note 2: Although a theoretical distinction may be made between the field effect and 15 the inductive effect as models for the Coulomb interaction between a given site and a 16 remote monopole or dipole within the same entity, the experimental distinction between 17 the two effects has proved difficult, because the field effect and the inductive effect are 18 ordinarily influenced in the same direction by structural changes. 19

Note 3: The substituent acts through the electric field that it generates, and it is an 20 oversimplification to reduce that interaction to that of a monopole or dipole. 21

See also electronic effect, inductive effect, polar effect. 22 See [172,191,199,200]. 23 rev[3] 24

25 flash photolysis 26 Spectroscopic or kinetic technique in which an ultraviolet, visible, or infrared radiation 27 pulse is used to produce transient species. 28

Note 1: Commonly, an intense pulse of short duration is used to produce a sufficient 29 concentration of transient species, suitable for spectroscopic observation. The most 30 common observation is of the absorption of the transient species (transient absorption 31 spectroscopy). 32

Note 2: If only photophysical processes are involved, a more appropriate term would 33 be “pulsed photoactivation”. The term “flash photolysis” would be correct only if 34 chemical bonds are broken (the Greek “lysis” means dissolution or decomposition and 35 in general lysis is used to indicate breaking). However, historically, the name has been 36 used to describe the technique of pulsed excitation, independently of the process that 37 follows the excitation. 38

See [9]. 39 40

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flash vacuum pyrolysis (FVP) 1 Thermal reaction of a molecule by exposure to a short thermal shock at high 2 temperature, usually in the gas phase. 3

Example: 4

See [201,202,203,204]. 5 rev[3] 6

7 fluxionality 8 Property of a chemical species that undergoes rapid degenerate rearrangements 9 (generally detectable by methods that allow the observation of the behaviour of 10 individual nuclei in a rearranged chemical species, e.g., NMR, X-ray). 11

Example: tricyclo[3.3.2.02,8]deca-3,6,9-triene (bullvalene), which has 1 209 600 (= 12 10!/3) interconvertible arrangements of the ten CH groups. 13 14

15 Note 1: Fluxionality differs from resonance, where no rearrangement of nuclear 16

positions occurs. 17 Note 2: The term is also used to designate positional change among ligands of 18

complex compounds and organometallics. In these cases the change is not necessarily 19 degenerate. 20

See also valence tautomerization. 21 rev[3] 22

23 force-field calculations 24

See molecular mechanics calculation. 25 GB 26

27 formal charge 28 Quantity (omitted if zero) attached to each atom in a Lewis structure according to 29 30

Zformal = Nvalence – Nlonepairs – ½ Nbonds 31 32

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Examples: CH2=N+=N–, H3O+, (CH3)2C=N-O– 1 2

Note: This formalism assumes that electrons in bonds are shared equally, regardless 3 of electronegativity. 4 5 Förster resonance-energy transfer (FRET) 6 dipole-dipole excitation transfer 7 Nonradiative mechanism for transfer of electronic excitation energy from one molecular 8 entity to another, distant one. It arises from the interaction between the transition dipole 9 moments of the two entities. 10

See [9]. 11 12 fractionation factor, isotopic 13 Ratio [x1(A)/x2(A)}/[x1(B)/x2(B)], where x is the mole fraction of the isotope designated 14 by the subscript, when the two isotopes are equilibrated between two different chemical 15 species A and B (or between specific sites A and B in the same chemical species). 16

Note 1: The term is most commonly met in connection with deuterium solvent isotope 17 effects, where the fractionation factor, symbolized by f , expresses the ratio 18

f = [xD(solute)/xH(solute)}/[xD(solvent)/xH(solvent)] 19 for the exchangeable hydrogen atoms in the chemical species (or sites) concerned. 20

Note 2: The concept is also applicable to transition states. 21 See [2]. 22 [3] 23

24 fragmentation 25 (1) Heterolytic or homolytic cleavage of a molecule into more than two fragments, 26 according to the general reaction (where a, b, c, d, and X are atoms or groups of atoms) 27 28

a–b–c–d–X → (a–b)+ + c=d + X– 29 or a–b–c–d–X → (a–b)· + c=d + X 30

Examples: 31 Ph3C–COOH + H+ → Ph3C+ + C=O + H2O 32 CH3N=NCH3 → CH3· + N2 + CH3 33

See [205]. 34 35 (2) Breakdown of a radical or radical ion into a closed-shell molecule or ion and a 36 smaller radical 37

Examples: 38 (CH3)3C–O· → (CH3)2C=O + H3C· 39 [ArBr]·– → Ar· + Br– (solution) 40

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[(CH3)3C–OH]·+ → (CH3)2C=OH+ + H3C· (gas-phase, following ionization) 1 [3] 2

3 Franck-Condon Principle 4 Approximation that an electronic transition is most likely to occur without change in 5 nuclear positions. 6

Note 1: The resulting state is called a Franck–Condon state, and the transition 7 involved is called a vertical transition. 8

Note 2: As a consequence, the intensity of a vibronic transition is proportional to the 9 square of the overlap integral between the vibrational wavefunctions of the two states 10 that are involved in the transition. 11

See [9]. 12 13 free energy 14 The thermodynamic function Gibbs energy (symbol G) or Helmholtz energy (symbol A) 15 specifically defined as 16 17

G = G(P,T) = H – TS 18 and A = A(V,T) = U – TS 19

20 where H is enthalpy, U is internal energy, and S is entropy. The possibility of 21 spontaneous motion for a statistical distribution of an assembly of atoms (at absolute 22 temperature T above 0 K) is governed by free energy and not by potential energy. 23

Note 1: The IUPAC recommendation is to use the specific terms Gibbs energy or 24 Helmholtz energy whenever possible. However, it is useful to retain the generic term 25 "free energy" for use in contexts where the distinction between (on the one hand) either 26 Gibbs energy or Helmholtz energy and (on the other hand) potential energy is more 27 important than the distinction between conditions either of constant pressure or of 28 constant volume; e.g., in computational modelling to distinguish between results of 29 simulations performed for ensembles under conditions of either constant P or constant 30 V at finite T and calculations based purely on potential energy. 31

Note 2: Whereas motion of a single molecular entity is determined by the force acting 32 upon it, which is obtained as the negative gradient of the potential energy, motion for 33 an assembly of many molecular entities is determined by the mean force acting upon 34 the statistical distribution, which is obtained as the negative gradient of the potential of 35 mean force. 36 37 free radical 38

See radical. 39 [3] 40

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1 frontier orbitals 2 Highest-energy Occupied Molecular Orbital (HOMO) and Lowest-energy Unoccupied 3 Molecular Orbital (LUMO) of a molecular entity. 4

Note 1: These terms should be limited to doubly occupied orbitals, and not to a singly 5 occupied molecular orbital (sometimes designated as a SOMO), because HOMO and 6 LUMO are ambiguous for molecular orbitals that are half filled and thus only partly 7 occupied or unoccupied. 8

Note 2: Examination of the mixing of frontier molecular orbitals of reacting molecular 9 entities affords an approach to the interpretation of reaction behaviour; this constitutes 10 a simplified perturbation theory of chemical behaviour. 11

Note 3: In some cases a subjacent orbital (Next-to-Highest Occupied Molecular 12 Orbital (NHOMO) or a Second Lowest Unoccupied Molecular Orbital (SLUMO)) may 13 affect reactivity. 14

See [8,206,207]. 15 See also SOMO, subjacent orbital. 16 rev[3] 17

18 frustrated Lewis acid-base pair 19 Acid and base for which adduct formation is prevented by steric hindrance. 20

21 See [208,209]. 22

23 fullerene 24 Molecular entity composed entirely of carbon, in the form of a hollow sphere, ellipsoid, 25 or tube. 26

Note: Spherical fullerenes are also called buckyballs, and cylindrical ones are called 27 carbon nanotubes or buckytubes. 28

See [40,210]. 29 30 functional group 31 Atom or group of atoms within molecular entities that are responsible for the 32 characteristic chemical reactions of those molecular entities. The same functional group 33 will undergo the same or similar chemical reaction(s) regardless of the size of the 34

N + B(CH3)3 N+ –B(CH3)3

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molecule it is a part of. However, its relative reactivity can be modified by nearby 1 substituents. 2

rev[3] 3 4 gas-phase acidity 5 Standard reaction Gibbs energy (∆rGº) change for the gas-phase reaction. 6

A–H ) A– + H+ 7

Note 1: The symbol often found in the literature is ∆acidG or GA. 8 Note 2: The corresponding enthalpy ∆rHo is often symbolized by ∆acidH and called 9

“enthalpy of acidity” or deprotonation enthalpy, abbreviated DPE. 10 See [211,212]. 11 rev[3] 12

13 gas-phase basicity 14 Negative of the standard reaction Gibbs energy (∆rGo) change for the gas-phase 15 reaction 16

B + H+ ) BH+ 17

Note: An acronym commonly used in the literature is GB. The corresponding 18 enthalpy ∆rHo is called proton affinity, PA, even though affinity properly refers to Gibbs 19 energy. Moreover, such acronyms are not accepted by IUPAC. 20

See [213,214]. 21 rev[3] 22

23 Gaussian orbital 24 Function centred on an atom of the form f(r) ∝!xiyjzkexp(–zr2), used to approximate 25 atomic orbitals in the LCAO-MO method. 26

See [8]. 27 28 geminate pair 29 Pair of molecular entities in close proximity within a solvent cage and resulting from 30 reaction (e.g., bond scission, electron transfer, group transfer) of a precursor. 31

Note: Because of the proximity the pair constitutes only a single kinetic entity. 32 See also ion pair, radical pair. 33 rev[3] 34

35 geminate recombination 36 Recombination reaction of a geminate pair. 37

Example: 38

39 RN=NR ∆ [R· N2 R·]cage R-R + N2

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rev[3] 1 2 3 general acid catalysis 4 Catalysis of a chemical reaction by Brønsted acids (which may include the solvated 5 hydrogen ion), where the rate of the catalysed part of the reaction is given by SHAkHA[HA] 6 multiplied by some function of substrate concentrations. 7

Note 1: General acid catalysis can be experimentally distinguished from specific 8 catalysis by hydrogen cations (hydrons) if the rate of reaction increases with buffer 9 concentration at constant pH and ionic strength. 10

Note 2: The acid catalysts HA are unchanged by the overall reaction. This 11 requirement is sometimes relaxed, but the phenomenon is then properly called pseudo-12 catalysis. 13

See also catalysis, catalytic coefficient, intramolecular catalysis, pseudo-catalysis, 14 specific catalysis. 15

rev[3] 16 17 general base catalysis 18 Catalysis of a chemical reaction by Brønsted bases (which may include the lyate ion), 19 where the rate of the catalysed part of the reaction is given by SBkB[B] multiplied by 20 some function of substrate concentrations. 21

See also general acid catalysis. 22 [3] 23

24 Gibbs energy diagram 25 Diagram showing the relative standard Gibbs energies of reactants, transition states, 26 reaction intermediates, and products, in the same sequence as they occur in a chemical 27 reaction. 28

Note 1: The abscissa expresses the sequence of reactants, products, reaction 29 intermediates, and transition states and is often an undefined "reaction coordinate" or 30 only vaguely defined as a measure of progress along a reaction path. In some 31 adaptations the abscissas are however explicitly defined as bond orders, Brønsted 32 exponents, etc. 33

Note 2: These points are often connected by a smooth curve (a "Gibbs energy 34 profile", commonly referred to as a "free-energy profile", a terminology that is 35 discouraged), but experimental observation can provide information on relative 36 standard Gibbs energies only at the maxima and minima and not at the configurations 37 between them. 38

Note 3: It should be noted that the use of standard Gibbs energies implies a common 39 standard state for all chemical species (usually 1 M for reactions in solution). Contrary 40

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to statements in many textbooks, the highest point on a Gibbs energy diagram does not 1 automatically correspond to the transition state of the rate-limiting step. For example, 2 in a stepwise reaction consisting of two elementary reaction steps 3

(1) A + B ⇄ C 4 (2) C + D → E 5

one of the two transition states must (in general) have a higher standard Gibbs energy 6 than the other. Under experimental conditions where all species have the standard-7 state concentration, then the rate-limiting step is that whose transition state is of highest 8 standard Gibbs energy. However, under (more usual) experimental conditions where 9 all species do not have the standard-state concentration, then the value of the 10 concentration of D determines which reaction step is rate-limiting. However, if the 11 particular concentrations of interest, which may vary, are chosen as the standard state, 12 then the rate-limiting step is indeed the one of highest Gibbs energy. 13 14

15 16

See also potential energy profile, potential-energy surface, reaction coordinate. 17 rev[3] 18

19 Gibbs energy of activation (standard free energy of activation) 20 ∆‡Gº 21 (SI unit: kJ mol–1) 22 Standard Gibbs energy difference between the transition state of an elementary 23 reaction and the ground state of the reactants for that step. It is calculated from the rate 24 constant k via the absolute rate equation: 25 26

&‡Gº = RT [ln( (kB/J K-1)/(h/J s) ) – ln(k/[k]/T/K)] 27

28

&‡Gº = RT [ln({kB}/{h}) – ln({k}/{T})] 29

30

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where kB is Boltzmann's constant, [k] are the units of k, and h Planck's constant. The 1 values of the rate constants, and hence the Gibbs energies of activation, depend upon 2 the choice of concentration units (or of the thermodynamic standard state). 3

Note 1: For a complex stepwise reaction, composed of many elementary reactions, 4 ∆‡Gº, the observed Gibbs energy of activation (activation free energy), can be 5 calculated as RT [ln(kB/J K-1)/(h/J s) – ln(k’/[k’]/T/K)], where k’ is the observed rate 6 constant, k’ should be divided by its units, and T/K is the absolute temperature, of 7 dimension 1, since the argument of a logarithmic function should be of dimension 1, as 8 expressed by the reduced variables in the second form of the equation. 9

Note 2: Both ∆‡Gº and k’ are non-trivial functions of the rate constants and activation 10 energies of the elementary steps. 11

See also enthalpy of activation, entropy of activation. 12 rev[3] 13

14 graphene 15 Allotrope of carbon, whose structure is a one-atom-thick planar sheet of sp2-bonded 16 carbon atoms in a honeycomb (hexagonal) crystal lattice. 17 18 ground state 19 State of lowest Gibbs energy of a system [2]. 20

Note: In photochemistry and quantum chemistry the lowest-energy state of a 21 chemical entity (ground electronic state) is usually meant. 22

See [9]. 23 See also excited state. 24 rev[3] 25

26 group 27

See functional group, substituent. 28 rev[3] 29

30 Grunwald-Winstein equation 31 Linear Gibbs-energy relation (Linear free-energy relation) 32 33

lg(kS/k0) = mY 34 35 expressing the dependence of the rate of solvolysis of a substrate on the ionizing power 36 of the solvent, where the rate constant k0 applies to the reference solvent (80:20 37 ethanol-water by volume) and kS to the solvent S. 38

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Note 1: The parameter m is characteristic of the substrate and is assigned the value 1 unity for tert-butyl chloride (2-chloro-2-methylpropane). The value Y is intended to be a 2 quantitative measure of the ionizing power of the solvent S. 3

Note 2: The equation was later extended [215] to the form 4 5

lg(kS/k0) = mY + lN 6 7 where N is the nucleophilicity of the solvent and l a susceptibility parameter 8

Note 3: The equation has also been applied to reactions other than solvolysis. 9 Note 4: For the definition of other Y-scales, see [216,217,218,219]. 10 See also Dimroth-Reichardt ET parameter, polarity. 11 rev[3] 12

13 guest 14 Organic or inorganic ion or molecule that occupies a cavity, cleft, or pocket within the 15 molecular structure of a host molecular entity and forms a complex with it or that is 16 trapped in a cavity within the crystal structure of a host, but with no covalent bond being 17 formed. 18

See also crown ether, cryptand, inclusion compound. 19 [3] 20

21 half-life 22 t1/2 23 (SI unit: s) 24 Time required for the concentration of a particular reacting chemical species to fall to 25 one-half of its initial value. 26

Note 1: Half-life is independent of initial concentration only for a first-order process. 27 Note 2: For first-order reactions t1/2 = t ln2, where t is the lifetime. 28 See also lifetime. 29 rev[3] 30

31 halochromism 32 Colour change that occurs on addition of acid or base to a solution of a compound as a 33 result of chemical reaction, or on addition of a salt as a result of changing the solvent 34 polarity. 35

See [220,221]. 36 [3] 37

38 halogen bond 39

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Association between a Lewis-acidic halogen atom in a molecular entity and a Lewis-1 basic region in another, or the same, molecular entity, which acts as an electron-pair 2 donor, such that the balance of forces of attraction and repulsion results in net 3 stabilization. 4

Note 1: Typical halogen-bond donors are I2, Br2, ICN, and ICΞCH. 5 Note 2: This is analogous to a hydrogen bond, in which the H is the acidic atom. 6 Note 3: The interaction provides a stabilization of a few kJ mol–1. 7 See [222,223,224,225]. 8

9 10 Hammett equation (Hammett relation) 11 Equation of the form 12 13

lg {kX} = r sX + lg {k0} 14 or lg {KX} = r sX + lg {K0} 15 16 expressing the influence of meta and para substituents X on the reactivity of the 17 functional group Y in the benzene derivatives m- and p-XC6H4Y, where {kX} and {KX} 18 are the numerical values of the rate or equilibrium constant, respectively, for the 19 reactions of m- and p-XC6H4Y, sX is the substituent constant characteristic of m- or p-20 X, and r is the reaction constant characteristic of the given reaction of Y. The values lg 21 {k0} and lg {K0} are intercepts for graphical plots of lg {kX} or lg {KX}, respectively, against 22 sX for all substituents X (including X = H, if present). 23

Note 1: It is very common in the literature to find either of these equations written 24 without the curly brackets denoting a reduced rate or equilibrium constant, i.e., k or K 25 divided by its unit, [k] or [K], respectively. 26

Note 2: The Hammett equation is most frequently used to obtain the value of r which, 27 as the slope of a graph or least-squares fit, is independent of the choice of units for kX 28 or KX. If the equation is used to estimate the unknown value of a rate or equilibrium 29 constant for an X-substituted compound based upon the known value of sX, the ordinate 30 must be multiplied by [kX] or [KX] in order to obtain the value with its appropriate units. 31

Note 3: The equation is often encountered in a form with kH or KH incorporated into 32 the logarithm on the left-hand side, where kH or KH corresponds to the reaction of parent 33 C6H5Y, with X = H; 34 35

lg(kX/kH) = r sX 36 or lg(KX/KH) = r sX 37 38 This form satisfies the requirement that arguments of the lg function must be of 39 dimension 1, but it would suggest a one-parameter linear least-squares fit, whereas lg 40

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k0, lg K0, lg k0/[k0] and lg K0/[K0], in the other forms correctly represent the intercept in 1 a two-parameter linear least-squares fit of lg k, lg K, lg k/[k], or lg K/[K] vs. sX. 2

See [21,22,126,191,226]. 3 See also extended Hammett equation, Taft equation, Yukawa-Tsuno equation, s-4

constant, r-value. 5 rev[3] 6

7 8 9 10 Hammond postulate (Hammond-Leffler principle) 11 Hypothesis that, when a transition state leading to a high-energy reaction intermediate 12 (or product) has nearly the same energy as that intermediate (or product), the two are 13 interconverted with only a small reorganization of molecular structure. 14

Note 1: Essentially the same idea is sometimes referred to as "Leffler's assumption", 15 namely, that the transition state bears a greater resemblance to the less stable species 16 (reactant or reaction intermediate/product). Many textbooks and physical organic 17 chemists, however, express the idea in Leffler's form (couched in terms of Gibbs 18 energies) but attribute it to Hammond (whose original conjecture concerns structure). 19

Note 2: As a corollary, it follows that a factor stabilizing a reaction intermediate will 20 also stabilize the transition state leading to that intermediate. 21

Note 3: If a factor stabilizes a reaction intermediate (or reactant or product), then the 22 position of the transition state along the minimum-energy reaction path (MERP) for that 23 elementary step moves away from that intermediate, as shown in the energy diagram 24 (a) below left, where the transition state moves from ‡ to ‡’ when the species to the 25 right is stabilized. This behaviour is often called a Hammond effect and is simply a 26 consequence of adding a linear perturbation to the parabola. 27 28

29 Note 4: If a structure lying off the MERP is stabilized, then the position of the 30

transition state moves toward that structure, as shown in the energy diagram (b) above 31

(a) (b)

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right, where the transition state moves from ‡ to ‡’. This behaviour is often called anti-1 Hammond and arises because the transition state is a maximum along the MERP but 2 a minimum perpendicular to it. 3

See [227,228,229,230]. 4 See Leffler's relation, More O'Ferrall – Jencks diagram, parallel effect, perpendicular 5

effect. 6 rev[3] 7

8 Hansch constant 9 Measure of the contribution of a substituent to the partition ratio of a solute, defined as 10 11

pX = lg(PX/PH) 12 13 where PX is the partition ratio for the compound with substituent X and PH is the partition 14 constant for the parent. 15

See [231,232]. 16 rev[3] 17

18 hapticity 19 Topological description for the number of contiguous atoms of a ligand that are bonded 20 to a central metal atom. 21

Note: The hapticity is indicated as a superscript following the Greek letter h. 22 Example: (C5H5)2Fe (ferrocene) = bis(h5-cyclopentadienyl)iron, where h5 can be read 23

as eta-five or pentahapto. 24 See [29]. 25

26 hard acid, base 27 Lewis acid with an acceptor centre or Lewis base with a donor centre (e.g., an oxygen 28 atom) of low polarizability. 29

Note 1: A high polarizability characterizes a soft acid or base. 30 Note 2: Whereas the definition above is qualitative, a theoretically consistent 31

definition of hardnessh, in eV, is as half the second derivative of the calculated energy 32 E with respect to N, the number of electrons, at constant potential n due to the nuclei. 33 34

𝜂 = 128

∂#𝐸∂𝑁#<

$ 35

36 Note 3: Other things being equal, complexes of hard acids with hard bases or of soft 37

acids with soft bases have an added stabilization (sometimes called the HSAB rule). 38 For limitations of the HSAB rule, see [36,233]. 39

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See [234,235,236]. 1 rev[3] 2

3 HBA solvent 4 Strongly or weakly basic solvent capable of acting as a hydrogen-bond acceptor (HBA) 5 and forming strong intermolecular solute-solvent hydrogen bonds. 6

Note: A solvent that is not capable of acting as a hydrogen-bond acceptor is called 7 a non-HBA solvent. 8

See [17]. 9 10 HBD solvent (Hydrogen-Bond Donating solvent, also dipolar HBD solvent and protic 11 solvent) 12 Solvent with a sizable permanent dipole moment that bears suitably acidic hydrogen 13 atoms to form strong intermolecular solvent-solute hydrogen bonds. 14

Note: A solvent that is not capable of acting as hydrogen-bond donor is called a non-15 HBD solvent (formerly aprotic solvent). 16

See [17,155]. 17 18 heat capacity of activation 19 CP‡ 20 (SI unit: J mol–1 K–1) 21 Temperature coefficient of ∆‡H (enthalpy of activation) or ∆‡S (entropy of activation) at 22 constant pressure according to the equations: 23 24

CP‡ = (∂∆‡H/∂T)P = T(∂∆‡S/∂T)P 25 26

See [237]. 27 rev[3] 28

29 Henderson-Hasselbalch equation 30 Equation of the form 31 32

pH = pKa – lg([HA]/[A–]) 33 34 relating the pH of a buffer solution to the ratio [HA]/[A–] and the dissociation constant of 35 the acid Ka. 36

See [238]. 37 rev[3] 38

39 heterobimetallic complex 40

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Metal complex having two different metal atoms or ions. 1 rev[3] 2

3 heteroleptic 4 Characteristic of a transition metal or Main Group compound having more than one type 5 of ligand. 6

See also homoleptic. 7 [3] 8

9 10 heterolysis, heterolytic bond fission 11 Cleavage of a covalent bond so that both bonding electrons remain with one of the two 12 fragments between which the bond is broken, e.g., 13

14 See also heterolytic bond-dissociation energy, homolysis. 15 [3] 16

17 heterolytic bond-dissociation energy 18 Energy required to break a given bond of a specific compound by heterolysis. 19

Note: For the dissociation of a neutral molecule AB in the gas phase into A+ and B– 20 the heterolytic bond-dissociation energy D(A+B–) is the sum of the homolytic bond-21 dissociation energy, D(A–B), and the adiabatic ionization energy of the radical A· minus 22 the electron affinity of the radical B·. 23

[3] 24 25 high-throughput screening 26 Automated method to quickly assay large libraries of chemical species for the affinity of 27 small organic molecules toward a target of interest. 28

Note: Currently more than 105 different compounds can be tested per day. 29 See [239]. 30

31 highest occupied molecular orbital (HOMO) 32 Doubly filled molecular orbital of highest energy. 33

Note: Examination of the HOMO can predict whether an electrocyclic reaction is 34 conrotatory or disrotatory. 35

See also frontier orbitals. 36 37 Hildebrand solubility parameter [symbol d, derived unit: Pa1/2 = (kg m–1 s–2)1/2] 38 Ability of a solvent to dissolve a non-electrolyte, defined as the square root of the 39 solvent's cohesive energy density (also called cohesive pressure, equal to the energy 40

A B A+ + B–

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of vaporization divided by the solvent’s molar volume and corresponding to the energy 1 necessary to create a cavity in the solvent). 2

See [240]. 3 rev[3] 4

5 Hofmann rule 6 Observation that when two or more alkenes can be produced in a b-elimination reaction, 7 the alkene having the smallest number of alkyl groups attached to the double bond 8 carbon atoms is the predominant product. 9

Note: This orientation is observed in elimination reactions of quaternary ammonium 10 salts and tertiary sulfonium (sulfanium) salts, and in certain other cases where there is 11 steric hindrance. 12

See [241]. 13 See also Saytzeff rule. 14 rev[3] 15

16 HOMO 17 (1) Acronym for Highest Occupied Molecular Orbital. 18

See also frontier orbitals. 19 (2) Prefix (in lower case) used to indicate a higher homologue of a compound, as 20 homocysteine for HSCH2CH2CH(NH2)COOH, the homologue of cysteine, 21 HSCH2CH(NH2)COOH. 22

rev[3] 23 24 homoaromatic 25 Showing features of aromaticity despite a formal discontinuity in the overlap of a cyclic 26 array of p orbitals resulting from the presence of an sp3-hybridized atom at one or 27 several positions within the cycle, in contrast to an aromatic molecule, where there is a 28 continuous overlap of p orbitals over a cyclic array. 29

Note 1: Homoaromaticity arises because p-orbital overlap can bridge an sp3 centre. 30 Note 2: Pronounced homoaromaticity is not normally associated with neutral 31

molecules, but mainly with ionic species, e.g., the "homotropylium" cation, C8H9+, 32

33 Note 3: In bis-, tris-, (etc.) homoaromatic species, two, three, (etc.) single sp3 centres 34

separately interrupt the π-electron system. 35 [3] 36

37

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homodesmotic reaction 1 Subclass of isodesmic reactions in which reactants and products contain not only the 2 same number of carbon atoms in each state of hybridization but also the same number 3 of each CHn group joined to n hydrogen atoms. 4

Examples: 5 6

cyclo-(CH2)3 + 3 CH3CH3 ⇄ 3 CH3CH2CH3 [3CH2, 6CH3] 7 C6H6 (benzene) + 3 CH2=CH2 ⇄ 3 CH2=CH–CH=CH2 [6CH, 6CH2] 8

9 Note: The definition may be extended to molecules with heteroatoms. 10 See [8]. 11 rev[3] 12

13 homoleptic 14 Characteristic of a transition-metal or Main Group compound having only one type of 15 ligand, e.g., Ta(CH3)5 16

See also heteroleptic. 17 [3] 18

19 homolysis 20 Cleavage of a bond so that each of the molecular fragments between which the bond 21 is broken retains one of the bonding electrons. 22

Note 1: A unimolecular reaction involving homolysis of a bond not forming part of a 23 cyclic structure in a molecular entity containing an even number of (paired) electrons 24 results in the formation of two radicals: 25

26 Note 2: Homolysis is the reverse of colligation. 27 See also bond dissociation energy, heterolysis. 28 [3] 29

30 host 31 Molecular entity that forms complexes (adducts) with organic or inorganic guests, or a 32 chemical species that can accommodate guests within cavities of its crystal structure. 33

Examples include cryptands and crown ethers (where there are ion/dipole attractions 34 between heteroatoms and cations), hydrogen-bonded molecules that form clathrates 35 (e.g., hydroquinone or water), and host molecules of inclusion compounds (e.g., urea 36 or thiourea), where intermolecular forces and hydrophobic interactions bind the guest 37 to the host molecule. 38

rev[3] 39

A B A• + B•

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1 Hückel molecular orbital (HMO) theory 2 Simplest molecular orbital theory of p-conjugated molecular systems. It uses the 3 following approximations: p-electron approximation; LCAO representation of the p 4 molecular orbitals; neglect of electron-electron and nuclear-nuclear repulsions. The 5 diagonal elements of the effective Hamiltonian (Coulomb integrals) and the off-diagonal 6 elements for directly bonded atoms (resonance integrals) are taken as empirical 7 parameters, all overlap integrals being neglected. 8

See [8]. 9 rev[3] 10

11 Hückel (4n + 2) rule 12 Principle that monocyclic planar (or almost planar) systems of trigonally (or sometimes 13 digonally) hybridized atoms that contain (4n + 2) p electrons (where n is an integer, 14 generally 0 to 5) exhibit aromatic character. 15

Note 1: This rule is derived from Hückel MO theory calculations on planar monocyclic 16 conjugated hydrocarbons (CH)m where integer m!is at least 3, according to which (4n + 17 2) electrons are contained in a closed-shell system. 18

Examples: 19

20 Note 2: Planar systems containing 4n p electrons (such as cyclobutadiene and 21

cyclopentadienyl cation) are antiaromatic. 22 Note 3: Cyclooctatetraene, with 8 p electrons, is nonplanar and therefore neither 23

aromatic nor antiaromatic but nonaromatic. 24 See [8]. 25 See also conjugation, Möbius aromaticity. 26 rev[3] 27

28 hybrid orbital 29 Atomic orbital constructed as a linear combination of atomic orbitals on an atom. 30

Note 1: Hybrid orbitals are often used to describe the bonding in molecules 31 containing tetrahedral (sp3), trigonal (sp2), and digonal (sp) atoms, whose s bonds are 32 constructed using 1:3, 1:2, or 1:1 combinations, respectively, of s and p atomic orbitals. 33

Note 2: Construction of hybrid orbitals can also include d orbitals, as on an octahedral 34 atom, with d2sp3 hybridization. 35

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Note 3: Integer ratios are not necessary, and the general hybrid orbital made from s 1 and p orbitals can be designated as spl. 2 3 hydration 4 Addition of water or of the elements of water (i.e., H and OH) to a molecular entity or to 5 a chemical species. 6

Example: hydration of ethene: 7 CH2=CH2 + H2O ® CH3CH2OH 8

Note: In contrast to aquation, hydration, as in the incorporation of waters of 9 crystallization into a protein or in the formation of a layer of water on a nonpolar surface, 10 does not necessarily require bond formation. 11

See [48]. 12 See also aquation. 13 rev[3] 14

15 hydrogen bond 16 An attractive interaction between a hydrogen atom from a molecule or a molecular 17 fragment X–H in which X is more electronegative than H, and an atom or a group of 18 atoms in the same or a different molecule, in which there is evidence of bond formation. 19

Note 1: Both electronegative atoms are usually (but not necessarily) from the first 20 row of the Periodic Table, i.e., N, O, or F. 21

Note 2: A hydrogen bond is largely an electrostatic interaction, heightened by the 22 small size of hydrogen, which permits proximity of the interacting dipoles or charges. 23

Note 3: Hydrogen bonds may be intermolecular or intramolecular. 24 Note 4: With few exceptions, usually involving hydrogen fluoride and fluoride and 25

other ions, the associated energies are less than 20-25 kJ mol-1. 26 Note 5: Hydrogen bonds are important for many chemical structures, giving rise to 27

the attraction between H2O molecules in water and ice, between the strands of DNA, 28 and between aminoacid residues in proteins. 29

See [242,243]. 30 rev[3] 31

32 hydrolysis 33 Solvolysis by water, generally involving the rupture of one or more bonds in the reactant 34 and involvement of water as nucleophile or base. 35

Example: CH3C(=O)OCH2CH3 + H2*!!+!!CH3C(=O)OH + $%3CH2OH 36

rev[3] 37 38 hydron 39

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General name for the ion H+ either in natural abundance or where it is not desired to 1 distinguish between the isotopes, as opposed to proton for 1H+, deuteron for 2H+ and 2 triton for 3H+. 3

See [244]. 4 [3] 5

6 hydronation 7 Attachment of the ion H+ either in natural abundance or where it is not desired to 8 distinguish between the isotopes. 9 10 hydrophilicity 11 Capacity of a molecular entity or of a substituent to undergo stabilizing interactions with 12 polar solvents, in particular with water and aqueous mixtures, to an extent greater than 13 with a nonpolar solvent. 14

rev[3] 15 16 hydrophobic interaction 17 Tendency of lipophilic hydrocarbon-like groups in solutes to form intermolecular 18 aggregates in an aqueous medium. 19

Note: The name arises from the attribution of the phenomenon to the apparent 20 repulsion between water and hydrocarbons. However, the phenomenon is more 21 properly attributed to the effect of the hydrocarbon-like groups to avoid disrupting the 22 favorable water-water interactions. 23

[3] 24 25 hyperconjugation 26 Delocalization of electrons between s bonds and a p network. 27

Note 1: The concept of hyperconjugation is often applied to carbenium ions and 28 radicals, where the interaction is between s bonds and an unfilled or partially filled p or 29 p orbital. Resonance illustrating this for the tert-butyl cation is: 30

31 Note 2: A distinction is made between positive hyperconjugation, as above, and 32

negative hyperconjugation, where the interaction is between a filled s or p orbital and 33

C CH

+H3CH3C H

HC C

H+

H3CH3C H

H

C CF

–HH H

HC C

F–

HH H

H

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adjacent antibonding s* orbitals, as for example in the fluoroethyl anion (2-fluoroethan-1 1-ide). 2 3

Note 3: Historically, conjugation involves only p bonds, and hyperconjugation is 4 considered unusual in involving s bonds. 5

See [245,246,247,248]. 6 See also s, p, delocalization. 7 rev[3] 8

9 hypercoordinated 10 Feature of a main-group atom with a coordination number greater than four. 11

Example: the pentacoordinate carbon in the carbonium ion CH5+, where three C–H 12 bonds may be regarded as two-electron bonds and the two electrons in the remaining 13 CH2 fragment are delocalized over three atoms. Likewise, both hydrogens in the CH2 14 fragment are hypercoordinated. 15

See [2,8]. 16 See also agostic. 17 rev[3] 18

19 hypervalency 20 Ability of an atom in a molecular entity to expand its valence shell beyond the limits of 21 the Lewis octet rule. 22

Examples: PF5, SO3, iodine(III) compounds, and pentacoordinate carbocations 23 (carbonium ions). 24

Note: Hypervalent compounds are more common for the second- and subsequent-25 row elements in groups 15-18 of the periodic table. A proper description of the 26 hypervalent bonding implies a transfer of electrons from the central (hypervalent) atom 27 to the nonbonding molecular orbitals of the attached ligands, which are usually more 28 electronegative. 29

See [8]. 30 See also valence. 31 rev[3] 32

33 hypsochromic shift 34 Shift of a spectral band to higher frequency (shorter wavelength) upon substitution or 35 change in medium. 36

Note: This is informally referred to as a blue shift and is opposite to a bathochromic 37 shift ("red shift"), but these historical terms are discouraged because they apply only to 38 visible transitions. 39

See [9]. 40

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rev[3] 1 2 identity reaction 3 Chemical reaction whose products are chemically identical with the reactants 4

Examples: 5 (i) bimolecular exchange reaction of CH3I with I– 6 (ii) proton transfer between NH4+ and NH3 7 (iii) electron transfer between manganate(VI) MnO42– and permanganate MnO4–. 8 See also degenerate rearrangement. 9 rev[3] 10

11 imbalance 12 Feature that reaction parameters characterizing different bond-forming or bond-13 breaking processes in the same reaction change to different extents as the transition 14 state is approached (along some arbitrarily defined reaction path). 15

Note: Imbalance is common in reactions such as elimination, addition, and other 16 complex reactions that involve proton (hydron) transfer. 17

Example: the nitroalkane anomaly, where the Brønsted b exponent for hydron 18 removal is smaller than the Brønsted a for the nitroalkane as acid, because of 19 imbalance between the extent of bond breaking and the extent of resonance 20 delocalization in the transition state. 21

See [249]. 22 See also Brønsted relation, synchronization (principle of nonperfect 23

synchronization), synchronous. 24 rev[3] 25

26 inclusion compound (inclusion complex) 27 Complex in which one component (the host) forms a cavity or, in the case of a crystal, 28 a crystal lattice containing spaces in the shape of long tunnels or channels in which 29 molecular entities of a second chemical species (the guest) are located. 30

Note: There is no covalent bonding between guest and host, the attraction being 31 generally due to van der Waals forces. If the spaces in the host lattice are enclosed on 32 all sides so that the guest species is "trapped" as in a cage, such compounds are known 33 as clathrates or cage compounds". 34

See [40]. 35 [3] 36

37 induction period 38 Initial slow phase of a chemical reaction whose rate later accelerates. 39

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Note: Induction periods are often observed with radical reactions, but they may also 1 occur in other reactions, such as those where a steady-state concentration of the 2 reactants is not established immediately. 3

See [13]. 4 [3] 5

6 inductive effect 7 Experimentally observable effect (on rates of reaction, etc.) of a substituent through 8 transmission of charge through a chain of atoms by electrostatics. 9

Note: Although a theoretical distinction may be made between the field effect and 10 the inductive effect as models for the Coulomb interaction between a given site within 11 a molecular entity and a remote monopole or dipole within the same entity, the 12 experimental distinction between the two effects has proved difficult (except for 13 molecules of peculiar geometry, which may exhibit "reversed field effects"), because 14 the inductive effect and the field effect are ordinarily influenced in the same direction by 15 structural changes. 16

See [171,172,250]. 17 See also field effect, polar effect. 18 rev[3] 19

20 inert 21 Stable and unreactive under specified conditions. 22

[3] 23 24 inhibition 25 Decrease in rate of reaction brought about by the addition of a substance (inhibitor), by 26 virtue of its effect on the concentration of a reactant, catalyst, or reaction intermediate. 27

For example, molecular oxygen or 1,4-benzoquinone can act as an inhibitor in many 28 chain reactions involving radicals as intermediates by virtue of its ability to act as a 29 scavenger toward those radicals. 30

Note: If the rate of a reaction in the absence of inhibitor is vo and that in the presence 31 of a certain amount of inhibitor is v, the degree of inhibition (i) is given by 32 33

i = (vo – v)/vo 34 35

See also mechanism-based inhibition. 36 [3] 37

38 initiation 39

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Reaction or process generating radicals (or some other reactive reaction intermediates) 1 which then induce a chain reaction or catalytic cycle. 2

Example: In the chlorination of alkanes by a radical mechanism the initiation step 3 may be the dissociation of molecular chlorine. 4

rev[3] 5 6 inner-sphere (electron transfer) 7 Feature of an electron transfer between two metal centres that in the transition state 8 share a ligand or atom in their coordination shells. 9

Note: The definition has been extended to any situation in which the interaction 10 between the electron-donor and electron-acceptor centres in the transition state is 11 significant (>20 kJ mol–1). 12

See [9]. 13 See also outer-sphere electron transfer. 14 [3] 15

16 insertion 17 Chemical reaction or transformation of the general type 18 19

X–Z + Y ® X–Y–Z 20 21 in which the connecting atom or group Y replaces the bond joining the parts X and Z of 22 the reactant XZ. 23

Example: carbene insertion reaction 24 25

R3C–H + H2C: ® R3C–CH3 26 27

Note: The reverse of an insertion is called an extrusion. 28 [3] 29

30 intermediate (reactive intermediate) 31 Molecular entity in a stepwise chemical reaction with a lifetime appreciably longer than 32 a molecular vibration (corresponding to a local potential energy minimum of depth 33 greater than RT, and thus distinguished from a transition state) that is formed (directly 34 or indirectly) from the reactants and reacts further to give (directly or indirectly) the 35 products of a chemical reaction. 36

See also elementary reaction, reaction step, stepwise reaction. 37 [3] 38

39 intermolecular 40

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(1) Descriptive of any process that involves a transfer (of atoms, groups, electrons, etc.) 1 or interactions between two or more molecular entities. 2 (2) Relating to a comparison between different molecular entities. 3

See also intramolecular. 4 [3] 5

6 internal return 7

See ion-pair recombination. 8 rev[3] 9

10 intramolecular 11 (1) Descriptive of any process that involves a transfer (of atoms, groups, electrons, etc.) 12 or interactions between different parts of the same molecular entity. 13 (2) Relating to a comparison between different groups within the same molecular entity. 14

See also intermolecular. 15 [3] 16

17 intramolecular catalysis 18 Acceleration of a chemical transformation at one site of a molecular entity through the 19 involvement of another functional ("catalytic") group in the same molecular entity, 20 without that group appearing to have undergone change in the reaction product. 21

Note 1: The use of the term should be restricted to cases for which analogous 22 intermolecular catalysis by a chemical species bearing that catalytic group is 23 observable. 24

Note 2: Intramolecular catalysis can be detected and expressed in quantitative form 25 by a comparison of the reaction rate with that of a comparable model compound in 26 which the catalytic group is absent, or by measurement of the effective molarity of the 27 catalytic group. 28

See also effective molarity, neighbouring group participation. 29 [3] 30

31 intrinsic barrier, Δ‡G0 32 Gibbs energy of activation in the limiting case where ΔrGo = 0, i.e., when the effect of 33 thermodynamic driving force is eliminated, as in an identity reaction, X* + AX → X*A + 34 X, where A may be an atom, ion, or group of atoms or ions, and where the equilibrium 35 constant K is equal to 1. 36

Note: According to the Marcus equation, originally developed for outer-sphere 37 electron transfer reactions, the intrinsic barrier is related to λ, the reorganization energy 38 of the reaction, by the equation 39 40

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Δ‡Gº = l/4 1 2 For a non-identity reaction, Y + AX → YA + X, the intrinsic barrier Δ‡Gº(Y,X) is estimated 3 as ½[Δ‡G(X,X) + Δ‡G(Y,Y)], where the latter two terms are the Gibbs energies of 4 activation of the identity reactions X* + AX → X*A + X and Y* + AY → Y*A + Y, 5 respectively. 6

See [251,252,253,254]. 7 rev[3] 8

9 intrinsic reaction coordinate (IRC) 10 Minimum-energy path leading from the saddle point (corresponding to the transition 11 structure) on the potential-energy surface for an elementary reaction, obtained by 12 tracing the steepest descent in mass-weighted coordinates in both directions. 13

Note: The IRC is mathematically well defined, in contrast to the (generally) vague 14 reaction coordinate. Strictly, the IRC is a specific case of a minimum-energy reaction 15 path, and its numerical value at any point along this path is usually taken to be zero at 16 the saddle point, positive in the direction of the products, and negative in the direction 17 of the reactants. 18

See [8,255]. 19 rev[3] 20

21 inverted micelle (reverse micelle) 22 Association colloid formed reversibly from surfactants in non-polar solvents, leading to 23 aggregates in which the polar groups of the surfactants are concentrated in the interior 24 and the lipophilic groups extend toward and into the non-polar solvent. 25

Note: Such association is often of the type 26 Monomer ⇄ Dimer ⇄ Trimer ⇄ ··· ⇄ n-mer 27

and a critical micelle concentration is consequently not observed. 28 [3] 29

30 ion pair 31 Pair of oppositely charged ions held together by coulomb attraction without formation 32 of a covalent bond. 33

Note 1: Experimentally, an ion pair behaves as one unit in determining conductivity, 34 kinetic behaviour, osmotic properties, etc. 35

Note 2: Following Bjerrum, oppositely charged ions with their centres closer together 36 than a distance 37 38

d = *)*#+*

,π.+.r/B0 39

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1 are considered to constitute an ion pair. Here Z+ and Z– are the charge numbers of the 2 ions, e is the elementary charge, ε0 is the vacuum permittivity, εr is the relative 3 permittivity ("dielectric constant") of the medium, kB is Boltzmann's constant, and T is 4 the absolute temperature. This is the distance at which the Coulomb energy equals the 5 thermal energy, and e2/(4pe0kB) is approximately equal to 1.671 ´ 10-5 m K-1. 6

Note 3: An ion pair, the constituent ions of which are in direct contact (and not 7 separated by intervening solvent or by other neutral molecule) is designated as a "tight 8 ion pair" (or "intimate" or "contact ion pair"). A tight ion pair of R+ and X– is symbolically 9 represented as R+X–. 10

Note 4: By contrast, an ion pair whose constituent ions are separated by one or 11 several solvent or other neutral molecules is described as a "loose ion pair", 12 symbolically represented as R+||X–. The components of a loose ion pair can readily 13 interchange with other free or loosely paired ions in the solution. This interchange may 14 be detectable (e.g., by isotopic labelling) and thus afford an experimental distinction 15 between tight and loose ion pairs. 16

Note 5: A further conceptual distinction has sometimes been made between two 17 types of loose ion pairs. In "solvent-shared ion pairs" for which the ionic constituents of 18 the pair are separated by only a single solvent molecule, whereas in "solvent-separated 19 ion pairs" more than one solvent molecule intervenes. However, the term "solvent-20 separated ion pair" must be used and interpreted with care since it has also widely been 21 used as a less specific term for "loose" ion pair. 22

See [256]. 23 See also common-ion effect, dissociation, ion-pair return, special salt effect. 24 [3] 25

26 ion-pair recombination (formerly ion-pair return) 27 Recombination of a pair of ions R+ and X– formed from ionization of RX. 28

Note: Ion-pair recombination can be distinguished as external or internal, depending 29 on whether the ion pair did or did not undergo dissociation to free ions. 30

See ion pair. 31 rev[3] 32

33 ionic liquid (ionic solvent, molten salt) 34 Liquid that consists exclusively or almost exclusively of equivalent amounts of 35 oppositely charged ions. 36

Note 1: In practice the ions are monocations and monoanions. 37 Note 2: Ionic liquids that are liquid at or around room temperature are called room-38

temperature ionic liquids (RTILs). 39

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Note 3: The term ionic liquid has been often restricted to those water-free liquids that 1 have melting points (or glass-transition temperatures) below 100 °C, following a 2 definition given by Walden [257], who prepared the first ionic liquid, ethylammonium 3

nitrate, CH3CH2NH3+ NO3!, mp. 13-14 °C, for conductivity measurements. 4

Note 4: The terminology for ionic liquids is not yet settled, as stated by Welton [258]. 5 Room-temperature ionic liquid, non-aqueous ionic liquid, molten salt, liquid organic salt, 6 and fused salt are often synonymous. 7

See [259,260,261]. 8 9 ionic strength 10 I 11 (In concentration basis, Ic, SI unit: mol m–3, more commonly mol dm–3 or mol L–1, in 12 molality basis, Im , SI unit: mol kg-1) 13 In concentration basis: Ic = 0.5 SiciZi2, in which ci is the concentration of a fully 14 dissociated electrolyte in solution and Zi the charge number of ionic species i. 15 In molality basis: Im = 0.5 SimiZi2, in which mi is the molality of a fully dissociated 16 electrolyte in solution and Zi the charge number of ionic species i. 17

rev[3] 18 19 ionization 20 Generation of one or more ions. 21

Note 1: This may occur, e.g., by loss or gain of an electron from a neutral molecular 22 entity, by the unimolecular heterolysis of that entity into two or more ions, or by a 23 heterolytic substitution reaction involving neutral molecules, such as 24

M → M+• + e– (photoionization, electron ionization in mass spectrometry) 25 SF6 + e– → SF6–• (electron capture) 26 Ph3CCl → Ph3C+ Cl– (ion pair, which may dissociate to free ions) 27 CH3COOH + H2O → H3O+ + CH3CO2– (dissociation) 28

Note 2: In mass spectrometry ions may be generated by several methods, including 29 electron ionization, photoionization, laser desorption, chemical ionization, and 30 electrospray ionization. 31

Note 3: Loss of an electron from a singly, doubly, etc. charged cation is called 32 second, third, etc. ionization. 33

See also dissociation, ionization energy. 34 rev[3] 35

36 ionization energy 37 Ei 38 (SI unit J) 39

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Minimum energy required to remove an electron from an isolated molecular entity (in 1 its vibrational ground state) in the gaseous phase. 2

Note 1: If the resulting molecular entity is in its vibrational ground state, the energy 3 is the "adiabatic ionization energy". 4

Note 2: If the molecular entity produced possesses the vibrational energy determined 5 by the Franck-Condon principle (according to which the electron ejection takes place 6 without an accompanying change in molecular geometry), the energy is the "vertical 7 ionization energy". 8

Note 3: The name ionization energy is preferred to the somewhat misleading earlier 9 name "ionization potential". 10

See also ionization. 11 [3] 12

13 ionizing power 14 Tendency of a particular solvent to promote ionization of a solute. 15

Note: The term has been used in both kinetic and thermodynamic contexts. 16 See also Dimroth-Reichardt ET parameter, Grunwald-Winstein equation, Z-value. 17 [3] 18

19 ipso attack 20 Attachment of an entering group to a position in an aromatic compound already carrying 21 a substituent group other than hydrogen. 22

Note: The entering group may displace that substituent group or may itself be 23 expelled or migrate to a different position in a subsequent step. The term "ipso-24 substitution" is not used, since it is synonymous with substitution. 25

Example: 26

27 where E+ is an electrophile and Z is a substituent other than hydrogen. 28

See [262]. 29 See also cine-substitution, tele-substitution. 30 [3] 31

32 isentropic 33

See isoentropic. 34 35 isodesmic 36 Property of a reaction (actual or hypothetical) in which the types of bonds that are made 37 in forming the products are the same as those that are broken in the reactants. 38

ZZE

+E+ +

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Examples: 1

(i) PhCOOH + p-ClC6H4CO2– ! PhCO2– + p-ClC6H4COOH (1) 2

(ii) 2 CH3F ! CH2F2 + CH4 (2) 3 Note 1: Such processes have advantages for theoretical treatment. 4 Note 2: The Hammett equation as applied to equilibria, as in (1), succeeds because 5

it deals with isodesmic processes. 6 Note 3: For the use of isodesmic processes in quantum chemistry, see [263]. 7 See [8]. 8 See also homodesmotic. 9 rev[3] 10

11 isoelectronic 12 Having the same number of valence electrons and the same structure, i.e., number and 13 connectivity of atoms, but differing in some or all of the elements involved. 14

Examples: 15 (i) CO, N2, and NO+ 16 (ii) CH2=C=O and CH2=N+=N– 17 [3] 18

19 isoentropic 20 Isentropic 21 Property of a reaction series in which the individual reactions have the same standard 22 entropy or entropy of activation. 23

[3] 24 25 isoequilibrium relationship 26 Feature of a series of related substrates or of a single substrate under a series of 27 reaction conditions whereby the enthalpies and entropies of reaction can be correlated 28 by the equation 29 30

DrH –b DrS = constant 31 32

Note: The parameter b is called the isoequilibrium temperature. 33 See [264,265]. 34 See also compensation effect, isokinetic relationship. 35 [3] 36

37 isokinetic relationship 38

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Feature of a series of related substrates or of a single substrate under a series of 1 reaction conditions whereby the enthalpies of activation and entropies of activation can 2 be correlated by the equation 3

D‡H –b D‡S = constant 4 Note 1: The parameter b is called the isokinetic temperature. At this temperature all 5

members of the series react at the same rate. 6 Note 2: Isokinetic relationships as established by direct correlation of D‡H with D‡S 7

are often spurious, and the calculated value of b is meaningless, because errors in D‡H 8 lead to compensating errors in D‡S. Satisfactory methods of establishing such 9 relationships have been devised. 10

See [264,265]. 11 See also compensation effect, isoequilibrium relationship, isoselective relationship. 12 [3] 13

14 isolobal 15 Feature of two molecular fragments for which the number, symmetry properties, 16 approximate energy, shape of the frontier orbitals, and number of electrons in them are 17 similar. 18

Example: 19

20 See also isoelectronic. 21 rev[3] 22

23 isomer 24 One of several species (or molecular entities) all of which have the same atomic 25 composition (molecular formula) but differ in their connectivity or stereochemistry and 26 hence have different physical and/or chemical properties. 27

Note: Conformational isomers that interconvert by rapid rotation about single bonds 28 and configurations that interconvert by rapid pyramidal inversion are often not 29 considered as separate isomers. 30

rev[3] 31 32 isomerization 33 Chemical reaction in which the product is an isomer of the reactant. 34

See also molecular rearrangement. 35 rev[3] 36

37

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isosbestic point 1 Wavelength (or frequency) at which two or more components in a mixture have the 2 same molar absorption coefficients. 3

Note 1: Isosbestic points are commonly met when electronic spectra are taken (a) 4 on a solution in which a chemical reaction is in progress (in which case the two 5 absorbing components concerned are a reactant and a product, A → B), or (b) on a 6 solution in which the two absorbing components are in equilibrium and their relative 7 proportions are controlled by the concentration of some other component, typically the 8 concentration of hydrogen ions, as in an acid-base indicator equilibrium. 9

Note 2: The effect may also appear (c) in the spectra of a set of solutions of two or 10 more non-interacting components having the same total concentration. 11

Note 3: In all these examples, A (and/or B) may be either a single chemical species 12 or a mixture of chemical species present in invariant proportion. 13

Note 4: If absorption spectra of the types considered above intersect not at one or 14 more isosbestic points but over a progressively changing range of wavelengths, this is 15 prima facie evidence (a) for the formation of a reaction intermediate in substantial 16 concentration (A → C → B) or (b) for the involvement of a third absorbing species in 17 the equilibrium, e.g., 18

A ⇄ B + H+aq ⇄ C + 2 H+aq 19 or (c) for some interaction of A and B, e.g., 20

A + B ⇄ C 21 Note 5: Isobestic is a misspelling and is discouraged. 22 rev[3] 23

24 isoselective relationship 25 Relationship analogous to the isokinetic relationship, but applied to selectivity data of 26 reactions. 27

Note: At the isoselective temperature, the selectivities of the series of reactions 28 following the relationship are identical, within experimental error. 29

See [266]. 30 See also isoequilibrium relationship, isokinetic relationship. 31 [3] 32

33 isotope effect 34 Relative difference between, or the ratio of, the rate coefficients or equilibrium constants 35 of two reactions that differ only in the isotopic composition of one or more of their 36 otherwise chemically identical components. 37

Note: The ratio kl/kh of rate constants (or Kl/Kh of equilibrium constants) for “light” and 38 “heavy” reactions is most often used in studies of chemical reaction mechanisms. 39 However, the opposite ratios are frequently used in environmental chemistry and 40

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geochemistry; i.e., kh/kl or Kh/Kl. Furthermore the relative difference kh/kl – 1 or Kh/Kl – 1 1 (often expressed as the percentage deviation of the ratio from unity) is commonly 2 used to quantify isotopic fractionation. 3

See kinetic isotope effect, equilibrium isotope effect. 4 rev[3] 5

6 isotope effect, equilibrium 7 isotope effect, thermodynamic 8 Effect of isotopic substitution on an equilibrium constant 9

Note 1: For example, the effect of isotopic substitution in reactant A that participates 10 in the equilibrium: 11

A + B ⇄ C 12 is the ratio Kl/Kh of the equilibrium constant for the reaction in which A contains the light 13 isotope to that in which it contains the heavy isotope. The ratio can also be expressed 14 as the equilibrium constant for the isotopic exchange reaction: 15

Al + Ch ⇄ Ah + Cl 16 in which reactants such as B that are not isotopically substituted do not appear. 17

Note 2: The potential-energy surfaces of isotopic molecules are identical to a high 18 degree of approximation, so thermodynamic isotope effects can arise only from the 19 effect of isotopic mass on the nuclear motions of the reactants and products, and can 20 be expressed quantitatively in terms of nuclear partition functions: 21 22

Kl/Kh = [Ql(C)/Qh(C)]/ [Ql(A)/Qh(A)] 23 24

Note 3: Although the nuclear partition function is a product of the translational, 25 rotational, and vibrational partition functions, the isotope effect is usually determined 26 almost entirely by the last named, specifically by vibrational modes involving motion of 27 isotopically different atoms. In the case of light atoms (i.e., protium vs. deuterium or 28 tritium) at moderate temperatures, the isotope effect is dominated by zero-point energy 29 differences. 30

See [267]. 31 See also fractionation factor. 32 rev[3] 33

34 isotope effect, heavy-atom 35

Isotope effect due to isotopes other than those of hydrogen. 36 [3] 37

38 isotope effect, intramolecular 39

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Kinetic isotope effect observed when a single substrate, in which the isotopic atoms 1 occupy equivalent reactive positions, reacts to produce a non-statistical distribution of 2 isotopologue products. 3 4

Example: PhCH2D + Br· → BrH + PhCHD· vs. BrD + PhCH2· 5 6 The intramolecular isotope effect kH/kD can be measured from the D content of product 7 (PhCH2Br + PhCHDBr), which is experimentally much easier than measuring the 8 intermolecular isotope effect kH/kD from the separate rates of reaction of PhCH3 and 9 PhCD3. 10

rev[3] 11 12 isotope effect, inverse 13 Kinetic isotope effect in which kl/kh < 1, i.e., the heavier substrate reacts more rapidly 14 than the lighter one, as opposed to the more usual "normal" isotope effect, in which kl/kh 15 > 1. 16

Note: The isotope effect will be "normal" when the vibrational frequency differences 17 between the isotopic transition states are smaller than in the reactants. Conversely, an 18 inverse isotope effect can be taken as evidence for an increase in the force constants 19 on passing from the reactant to the transition state. 20

[3] 21 22 isotope effect, kinetic 23 Effect of isotopic substitution on a rate constant. 24

For example in the reaction 25 A + B → C 26

the effect of isotopic substitution in reactant A is expressed as the ratio of rate 27 constants kl/kh, where the superscripts l and h represent reactions in which the 28 molecules A contain the light and heavy isotopes, respectively. 29

Note 1: Within the framework of transition-state theory, where the reaction is 30 rewritten as 31

A + B ⇄ [TS]‡ → C 32 kl/kh can be regarded as if it were the equilibrium constant for an isotope exchange 33 reaction between the transition state [TS]‡ and the isotopically substituted reactant A, 34 and calculated from their vibrational frequencies as in the case of a thermodynamic 35 isotope effect (see thermodynamic (equilibrium) isotope effect). 36

Note 2: Isotope effects like the above, involving a direct or indirect comparison of the 37 rates of reaction of isotopologues, are called "intermolecular", in contrast to 38 intramolecular isotope effects (see intramolecular isotope effect), in which a single 39

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substrate reacts to produce a non-statistical distribution of isotopologue product 1 molecules. 2

See [267,268]. 3 [3] 4

5 isotope effect, primary 6 Kinetic isotope effect attributable to isotopic substitution of an atom to which a bond is 7 made or broken in the rate-limiting step or in a pre-equilibrium step of a reaction. 8

Note: The corresponding isotope effect on the equilibrium constant of a reaction in 9 which one or more bonds to isotopic atoms are broken is called a primary equilibrium 10 isotope effect. 11

See also secondary isotope effect. 12 [3] 13

14 isotope effect, secondary 15 Kinetic isotope effect that is attributable to isotopic substitution of an atom to which 16 bonds are neither made nor broken in the rate-limiting step or in a pre-equilibrium step 17 of a specified reaction. 18

Note 1: The corresponding isotope effect on the equilibrium constant of such a 19 reaction is called a secondary equilibrium isotope effect. 20

Note 2: Secondary isotope effects can be classified as a, b, etc., where the label 21 denotes the position of isotopic substitution relative to the reaction centre. 22

Note 3: Although secondary isotope effects have been discussed in terms of 23 conventional electronic effects, e.g., induction, hyperconjugation, hybridization, such an 24 effect is not electronic but vibrational in origin. 25

See [269]. 26 See also steric isotope effect. 27 rev[3] 28

29 isotope effect, solvent 30 Kinetic or equilibrium isotope effect resulting from change in the isotopic composition 31 of the solvent. 32

See also kinetic isotope effect, equilibrium isotope effect. 33 [3] 34

35 isotope effect, steric 36 Secondary isotope effect attributed to the different vibrational amplitudes of 37 isotopologues. 38

Note 1: For example, both the mean and mean-square amplitudes of vibrations 39 associated with C–H bonds are greater than those of C–D bonds. The greater effective 40

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bulk of molecules containing the former may be manifested by a steric effect on a rate 1 or equilibrium constant. 2

Note 2: Ultimately the steric isotope effect arises from changes in vibrational 3 frequencies and zero-point energies, as normally do other isotope effects. 4

rev[3] 5 6 isotope effect, thermodynamic 7

See isotope effect, equilibrium. 8 [3] 9

10 isotope exchange 11 Chemical reaction in which the reactant and product chemical species are chemically 12 identical but have different isotopic composition. 13

Note: In such a reaction the isotope distribution tends towards equilibrium (as 14 expressed by fractionation factors) as a result of transfers of isotopically different atoms 15 or groups. For example, 16

[3] 17 18 isotopic perturbation, method of 19 Measurement of the NMR shift difference due to the isotope effect on a fast 20 (degenerate) equilibrium between two species that are equivalent except for isotopic 21 substitution. 22

Note: This method can distinguish a rapidly equilibrating mixture with time-averaged 23 symmetry from a single structure with higher symmetry. 24

See [270,271,272,273]. 25 [3] 26

27 isotopic scrambling 28 Achievement of, or the process of achieving, a redistribution of isotopes within a 29 specified set of atoms in a chemical species or group of chemical species. 30

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Examples 1

2 (where * denotes the position of an isotopically different atom.) 3

See also fractionation factor. 4 [3] 5

6 isotopologues 7 Molecular entities that differ only in isotopic composition (number of isotopic 8 substitutions), e.g., CH4, CH3D, CH2D2, 9

Note: These are isotopic homologues. It is a misnomer to call them isotopomers, 10 because they are not isomers with the same atoms. 11

rev[3] 12 13 isotopomers 14 Isomers having the same number of each isotopic atom but differing in their positions. 15 The term is a contraction of "isotopic isomer". 16

Note: Isotopomers can be either constitutional isomers (e.g., CH2DCH=O and 17 CH3CD=O) or isotopic stereoisomers (e.g., (R)- and (S)-CH3CHDOH or (Z)- and (E)-18 CH3CH=CHD). 19

See [11]. 20 [3] 21

22 Kamlet-Taft solvent parameters 23 Quantitative measure of solvent polarity, based on the solvent’s hydrogen-bond donor, 24 hydrogen-bond acceptor, dipolarity/polarizability, and cohesive-pressure properties. 25

See [274,275,276,277,278,279]. 26 See solvent parameter. 27 rev[3] 28

29 Kaptein-Closs rules 30 Rules used to predict the sign of chemically induced dynamic nuclear polarization 31 (CIDNP) effects. 32

See [280,281,282]. 33

I NH2 NH2

Ph N N Ph N N Ph N N*

(a)* *

(b)

+ I–+*

* *++ ++

NH2–

NH3

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1 Kekulé structure 2 Representation of a molecular entity (usually aromatic) with fixed alternating single and 3 double bonds, in which interactions between multiple bonds are ignored. 4

Example: For benzene the Kekulé structures are 5

6 Note: The distinction among Lewis structure, Kekulé structure, and line formula is 7

now not generally observed, nor is the restriction to aromatic molecular entities. 8 See also non-Kekulé structure. 9 rev[3] 10

11 keto-enol tautomerization 12 Interconversion of ketone and enol tautomers by hydron migration, accelerated by acid 13 or base catalysis. 14

Example: 15

16 See also tautomerization. 17

18 kinetic ambiguity 19 deprecated 20

See kinetic equivalence. 21 22 kinetic control (of product composition) 23 Conditions (including reaction times) that lead to reaction products in a proportion 24 governed by the relative rates of the parallel (forward) reactions by which those 25 products are formed, rather than by the equilibrium constants. 26

See also thermodynamic control. 27 [3] 28

29 kinetic electrolyte effect 30 kinetic ionic-strength effect 31

OEt

OOH

OEt

OO

enol keto

CC

CCC

C

CC

CCC

C

and

HH

HH

H

HH

H

HH

H

H

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General effect of an added electrolyte (i.e., other than, or in addition to, that due to its 1 possible involvement as a reactant or catalyst) on the rate constant of a reaction in 2 solution. 3

Note 1: At low concentrations (when only long-range coulombic forces need to be 4 considered) the effect on a given reaction is determined only by the ionic strength of 5 the solution and not by the chemical identity of the ions. This concentration range is 6 roughly the same as the region of validity of the Debye-Hückel limiting law for activity 7 coefficients. At higher concentrations the effect of an added electrolyte depends also 8 on the chemical identity of the ions. Such specific actions can sometimes be interpreted 9 as the incursion of a reaction path involving an ion of the electrolyte as reactant or 10 catalyst, in which case the action is not properly to be regarded just as a kinetic 11 electrolyte effect. At higher concentrations the effect of an added electrolyte does not 12 necessarily involve a new reaction path, but merely the breakdown of the Debye-Hückel 13 law, whereby ionic activity coefficients vary with the ion. 14

Note 2: Kinetic electrolyte effects are also called kinetic salt effects. 15 Note 3: A kinetic electrolyte effect ascribable solely to the influence of the ionic 16

strength on activity coefficients of ionic reactants and transition states is called a primary 17 kinetic electrolyte effect. A kinetic electrolyte effect arising from the influence of the 18 ionic strength of the solution upon the pre-equilibrium concentration of an ionic species 19 that is involved in a subsequent rate-limiting step of a reaction is called a secondary 20 kinetic electrolyte effect. A common case is the secondary electrolyte effect on the 21 concentration of hydrogen ion (acting as catalyst) produced from the ionization of a 22 weak acid in a buffer solution. To eliminate the complication of kinetic electrolyte effects 23 in buffer solutions, it is advisable to maintain constant ionic strength. 24

See [283]. 25 See also common-ion effect, order of reaction. 26 [3] 27

28 kinetic equivalence 29 Property of two reaction schemes that imply the same rate law. 30

Example: Schemes (i) and (ii) for the formation of C from A under conditions that B 31 does not accumulate as a reaction intermediate: 32

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1 2 3 4 5 6 7 8 9 10 11 12 Both equations for d[C]/dt are of the form 13

14 where r and s are constants (sometimes called rate coefficients). The equations are 15 identical in their dependence on concentrations and do not distinguish whether HO– 16 catalyses the formation of B and its reversion to A, or is involved only in its further 17 transformation to C. The two schemes are therefore kinetically equivalent. 18

[3] 19 20 21 Koppel-Palm solvent parameters 22 Quantitative measure of solvent polarity, based on the solvent’s permittivity, refractive 23 index, basicity or nucleophilicity, and acidity or electrophilicity. 24

See [284]. 25 See solvent parameter. 26 rev[3] 27

28 Kosower Z value 29

See Z-value. 30 [3] 31

32 labile 33 Property of a chemical species that is relatively unstable and transient or reactive. 34

Note: This term must not be used without explanation of the intended meaning. 35 See also inert, persistent, reactivity, unreactive. 36 [3] 37

38

=d[C]dt

r [A][HO– ]1 + s[HO–]

(i) A k2

k–1 , HO–

=d[C]dt

k1k2[A][HO–]k2 + k–1[HO–]

(ii) A k2k1

k–1

=d[C]dt

k1k2[A][HO– ]k–1 + k2[HO– ]

CB

B C

HO–

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Laurence solvent parameters 1 Quantitative measures of solvent polarity, based on the solvent’s dispersion/induction 2 interactions, electrostatic interactions between permanent multipoles, solute Lewis 3 base/solvent Lewis acid interactions, and solute HBD/solvent HBA interactions. 4

See [285]. 5 6 least nuclear motion, principle of (hypothesis of least motion) 7 Hypothesis that, for given reactants, the reactions involving the smallest change in 8 nuclear positions will have the lowest energy of activation. 9

Note: The basis for this hypothesis is that the energy of a structural deformation 10 leading toward reaction is proportional to the sum of the squares of the changes in 11 nuclear positions, which holds only for small deformations and is therefore not always 12 valid. 13

See [8,286]. 14 rev[3] 15

16 leaving group 17 Species (charged or uncharged) that carries away the bonding electron pair when it 18 becomes detached from another fragment in the residual part of the substrate. 19

Example 1: In the heterolytic solvolysis of (bromomethyl)benzene (benzyl bromide) 20 in acetic acid 21 22

PhCH2Br + AcOH → PhCH2OAc + HBr 23 24 the leaving group is Br–. 25

Example 2: In the reaction 26 27

MeS– + PhCH2N+Me3 → MeSCH2Ph + NMe3 28 29 the leaving group is NMe3. 30

Note: The historical term “leaving group” is ambiguous, because in the heterolysis of 31 R–X, both R+ and X- are fragments that leave from each other. For that reason, X– (as 32 well as Br– and NMe3 in examples 1 and 2) can unambiguously be called nucleofuges, 33 whereas R+ is an electrofuge. 34

See also electrofuge, entering group, nucleofuge. 35 rev[3] 36

37 Leffler's relation 38 Leffler’s assumption 39 In a series of elementary reactions the changes in Gibbs activation energies are often 40 found to be proportional to the changes in Gibbs energies for the overall reaction. 41

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1 d D‡G = a DrG0 2

3 This relation was interpreted in terms of the simple assumption that a small change in 4 any transition-state property P‡ is a linear combination of changes in reactant- and 5 product-state properties, PR and PP. 6 7

dP‡ = a dPP + (1 –a) dPR 8 9 Within the limits of this assumption, the parameter a is an approximate measure of the 10 fractional displacement of the transition state along the minimum-energy reaction path 11 from reactants to products. 12

See [109]. 13 Note: There are many exceptions to the validity of Leffler’s assumption that a is a 14

measure of the position of the transition state. 15 See [287]. 16 See Hammond postulate. 17 rev[3] 18

19 left-to-right convention 20 Arrangement of the structural formulae of the reactants so that the bonds to be made 21 or broken form a linear array in which the electron pushing proceeds from left to right. 22

See [288]. 23 [3] 24

25 levelling effect 26 Tendency of a solvent to make all Brønsted acids whose acidity exceeds a certain value 27 appear equally acidic. 28

Note 1: This phenomenon is due to the complete transfer of a hydron to a Brønsted-29 basic solvent from a dissolved acid stronger than the conjugate acid of the solvent. The 30 only acid present to any significant extent in all such solutions is the lyonium ion. 31

Note 2: For example, the solvent water has a levelling effect on the acidities of HClO4, 32 HCl, and HI. Aqueous solutions of these acids at the same (moderately low) 33 concentrations have the same acidities. 34

Note 3: A corresponding levelling effect applies to strong bases in protogenic 35 solvents. 36

[3] 37 38 Lewis acid 39

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Molecular entity (and the corresponding chemical species) that is an electron–pair 1 acceptor and therefore able to react with a Lewis base to form a Lewis adduct by 2 sharing the electron pair furnished by the Lewis base. 3

Example: 4 5

Me3B (Lewis acid) + :NH3 (Lewis base)!→!Me3B––N+H3 (Lewis adduct) 6 7

See also coordination. 8 [3] 9

10 Lewis acidity 11 Thermodynamic tendency of a substrate to act as a Lewis acid. 12

Note 1: This property is defined quantitatively by the equilibrium constant or Gibbs 13 energy for Lewis adduct formation of a series of Lewis acids with a common reference 14 Lewis base. 15

Note 2. An alternative measure of Lewis acidity in the gas phase is the enthalpy of 16 Lewis adduct formation for that Lewis acid with a common reference Lewis base. 17

See also electrophilicity, Lewis basicity. 18 rev[3] 19

20 Lewis adduct 21 Adduct formed between a Lewis acid and a Lewis base. 22

[3] 23 24 Lewis base 25 Molecular entity (and the corresponding chemical species) able to provide a pair of 26 electrons and thus capable of coordination to a Lewis acid, thereby producing a Lewis 27 adduct. 28

[3] 29 30 Lewis basicity 31 Thermodynamic tendency of a substance to act as a Lewis base. 32

Note 1: This property is defined quantitatively by the equilibrium constant or Gibbs 33 energy of Lewis adduct formation for that Lewis base with a common reference Lewis 34 acid. 35

Note 2: An alternative measure of Lewis basicity in the gas phase is the enthalpy of 36 Lewis adduct formation for that Lewis base with a common reference Lewis acid. 37

See also donicity, Lewis acidity, nucleophilicity, proton affinity. 38 rev[3] 39

40

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Lewis structure 1 electron-dot structure, Lewis formula 2 Representation of molecular structure in which (a) nonbonded valence electrons are 3 shown as dots placed adjacent to the atoms with which they are associated and in 4 which (b) a pair of bonding valence electrons in a covalent bond is shown as two dots 5 between the bonded atoms, and in which (c) formal charges (e.g., +, –, 2+) are attached 6 to atoms to indicate the difference between the nuclear charge (atomic number) and 7 the total number of electrons associated with that atom, on the formal basis that bonding 8 electrons are shared equally between atoms they join. 9

Examples: 10

11 Note 1: A double bond is represented by two pairs of dots, and a triple bond by three 12

pairs, as in the last example above. 13 Note 2: Bonding pairs of electrons are usually denoted by lines, representing 14

covalent bonds, as in line formulas, rather than as a pair of dots, and the lone pairs are 15 sometimes omitted. 16

Examples: 17

18 Note 3: The distinction among Lewis structure, Kekulé structure, and line formula is 19

now not generally observed. 20 rev[3] 21

22 lifetime (mean lifetime) 23 t 24 Time needed for the concentration of a chemical species that decays in a first-order 25 process to decrease to 1/e of its original value. i.e., c(t = t) = c(t = 0)/e. 26

Note 1: Statistically, it represents the mean life expectancy of the species. 27 Note 2: Mathematically: t = 1/k = 1/(Siki) where ki is the first-order rate constant for 28

the i-th decay process of the species. 29 Note 3: Lifetime is sometimes applied to processes that are not first-order. However, 30

in such cases the lifetime depends on the initial concentration of the entity or of a 31 quencher and, therefore, only an initial lifetime can be defined. In this case, it should be 32 called decay time. 33

See [9]. 34 See also chemical relaxation, half-life, rate of reaction. 35 [3] 36

37

H–Cl Cl– H-C C-H

H:C:::C:HH:Cl: :Cl:

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ligand 1 Atom or group bound to a central atom in a polyatomic molecular entity (if it is possible 2 to indicate such a central atom). 3

See [29]. 4 Note 1:The term is generally used in connection with metallic central atoms. 5 Note 2: In biochemistry a part of a polyatomic molecular entity may be considered 6

central, and atoms, groups, or molecules bound to that part are considered ligands. 7 See [289], p. 335. 8 [3] 9

10 line formula 11 Two-dimensional representation of molecular entities in which atoms are shown joined 12 by lines representing single bonds (or multiple lines for multiple bonds), without any 13 indication or implication concerning the spatial direction of bonds. 14

Examples: methanol, ethene 15

16 See also condensed formula, Kekulé formula, Lewis formula, skeletal formula. 17 rev[3] 18

19 linear free-energy relation 20 linear Gibbs-energy relation 21

[3] 22 23 linear Gibbs-energy relation 24 linear free-energy relation (LFER) 25 Linear correlation between the logarithm of a rate constant or equilibrium constant for 26 a series of reactions and the logarithm of the rate constant or equilibrium constant for a 27 related series of reactions. 28

Typical examples of such relations are the Brønsted relation and the Hammett 29 equation (see also s-value). 30

Note: The name arises because the logarithm of the value of an equilibrium constant 31 (at constant temperature and pressure) is proportional to a standard Gibbs energy (free 32 energy) change, and the logarithm of the value of a rate constant is a linear function of 33 the Gibbs energy (free energy) of activation. 34

[3] 35 36 linear solvation-energy relationship (LSER) 37

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Application of solvent parameters in the form of a single- or multi-parameter equation 1 expressing the solvent effect on a given property: e.g., rate of reaction, equilibrium 2 constant, spectroscopic shift. 3

Note: The solvent effect may be estimated as a linear combination of elementary 4 effects on a given property P, relative to the property P0 in the reference solvent: 5 6

P – P0 = a A + b B + p DP… 7 8 where A, B, DP, etc. are acidity, basicity, dipolarity, etc. parameters, and a, b, p… are 9 the sensitivity of the property to each effect. 10

See Catalán solvent parameters, Dimroth-Reichardt ET parameter, Kamlet-Taft 11 solvent parameters, Koppel-Palm solvent parameters, Laurence solvent parameters, 12 Z-value. 13

rev[3] 14 15 line-shape analysis 16 Method for determination of rate constants for dynamic chemical exchange from the 17 shapes of spectroscopic signals, most often used in nuclear magnetic resonance 18 spectroscopy. 19

[3] 20 21 Lineweaver-Burk plot 22

See Michaelis-Menten kinetics. 23 [3] 24

25 lipophilic 26 Feature of molecular entities (or parts of molecular entities) that have a tendency to 27 dissolve in fat-like solvents (e.g., hydrocarbons). 28

See also hydrophilic, hydrophobic interaction. 29 rev[3] 30

31 London forces 32 dispersion forces 33 Attractive forces between molecules due to their mutual polarizability. 34

Note: London forces are the principal components of the forces between nonpolar 35 molecules. 36

See [290]. 37 See also van der Waals forces. 38 [3] 39

40

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lone (electron) pair (nonbonding electron pair) 1 Two spin-paired electrons localized in the valence shell on a single atom. 2

Note: In structural formulas lone pairs should be designated as two dots. 3 See also Lewis structure. 4 rev[3] 5

6 LUMO 7 Acronym for Lowest Unoccupied Molecular Orbital 8

See frontier orbitals. 9 rev[3] 10

11 lyate ion 12 Anion produced by hydron (proton, deuteron, triton) removal from a solvent molecule. 13

Example: the hydroxide ion is the lyate ion of water. 14 [3] 15

16 lyonium ion 17 Cation produced by hydronation (protonation, deuteronation, tritonation) of a solvent 18 molecule. 19

Example: CH3OH2+ is the lyonium ion of methanol. 20 See also onium ion. 21 [3] 22

23 24 25 macroscopic diffusion control 26

See mixing control. 27 [3] 28

29 magnetic equivalence 30 Property of nuclei that have the same resonance frequency in nuclear magnetic 31 resonance spectroscopy and also identical spin-spin interactions with each nucleus of 32 a neighbouring group. 33

Note 1: The spin-spin interaction between magnetically equivalent nuclei is not 34 manifested in the spectrum, and has no effect on the multiplicity of the respective NMR 35 signals. 36

Note 2: Magnetically equivalent nuclei are necessarily chemically equivalent, but the 37 reverse is not necessarily true. 38

[3] 39 40

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magnetization transfer 1 NMR method for determining kinetics of chemical exchange by perturbing the 2 magnetization of nuclei in a particular site or sites and following the rate at which 3 magnetic equilibrium is restored. 4

Note: The most common perturbations are saturation and inversion, and the 5 corresponding techniques are often called "saturation transfer" and "selective inversion-6 recovery". 7

See also saturation transfer. 8 [3] 9

10 Marcus equation 11 General expression that correlates the Gibbs energy of activation (D‡G) with the Gibbs 12 energy of the reaction (DrGº) 13 14

D‡G = (l/4)(1 + DrGº/l)2 = D‡Go + 1/2 DrGº + (DrGº)2/(16 D‡Go) 15 16 where l is the reorganization energy and D‡Go is the intrinsic barrier, with l = 4D‡Go. 17

Note: Originally developed for outer-sphere electron transfer reactions, the Marcus 18 equation applies to many atom- and group-transfer reactions. The Marcus equation 19 captures earlier ideas that reaction thermodynamics can influence reaction barriers: 20 e.g., the Brønsted relation (1926), the Bell-Evans-Polanyi principle (1936-38), Leffler’s 21 relation (1953), and the Hammond postulate (1955) [227]. It also implies that changes 22 in intrinsic barriers may dominate over changes of reaction Gibbs energies and thus 23 account for the fact that reaction rates may not be controlled by the relative 24 thermodynamic stabilities of the products. 25

See [291,292,293,294,295,296]. 26 rev[3] 27

28 Markovnikov (Markownikoff) rule 29 Statement of the common mechanistic observation that in electrophilic addition 30 reactions the more electropositive atom (or part) of a polar molecule becomes attached 31 to the carbon bearing more hydrogens. 32

Note 1: This rule was originally formulated by Markownikoff (Markovnikov) as "In the 33 addition of hydrogen halides to [unsaturated] hydrocarbons, the halogen atom becomes 34 attached to the carbon bearing the lesser number of hydrogen atoms". 35

Note 2: This rule can be rationalized as the addition of the more electropositive atom 36 (or part) of the polar molecule to the end of the multiple bond that would result in the 37 more stable carbenium ion (regardless of whether the carbenium ion is a stable 38 intermediate or a transient structure along the minimum-energy reaction path). 39

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Note 3: Addition in the opposite sense, as in radical addition reactions, is commonly 1 called anti-Markovnikov addition. 2

See [297]. 3 [3] 4

5 mass action, law of 6 Statement that the velocity of a reaction depends on the active mass, i.e., the 7 concentrations of the reactants. 8

Example: for an association reaction (1) and its reverse (2) 9 10

A + B ® AB (1) 11 AB ® A + B (2) 12

13 the forward velocity is v1 = k1 [A][B], with k1 the rate constant for the association reaction. 14 For the dissociation reaction 2 the velocity is v2 = k2 [AB]. This is valid only for 15 elementary reactions. Furthermore, the law of mass action states that, when a 16 reversible chemical reaction reaches equilibrium at a given temperature, the forward 17 rate is the same as the backward rate. Therefore, the concentrations of the chemicals 18 involved bear a constant relation to each other, described by the equilibrium constant, 19 i.e., for 20 21

A + B AB 22 23 in equilibrium, v1 = k1 [A][B] = v2 = k2 [AB] and one form of the equilibrium constant for 24 the above chemical reaction is the ratio 25 26

Kc = [AB][A][B]

= )')+

27

28 Note: First recognized in 1864 as the kinetic law of mass action by Guldberg and 29

Waage, who first introduced the concept of dynamic equilibrium, but incorrectly 30 assumed that the rates could be deduced from the stoichiometric equation [298]. Only 31 after the work of Horstmann [299] and van’t Hoff [300] a mathematical derivation of the 32 reaction rates considering the order of the reaction involved was correctly made. 33

See also [301,302,303]. 34 rev[3] 35

36 matrix isolation 37

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Technique for preparation of a reactive or unstable species by dilution in an inert solid 1 matrix (argon, nitrogen, etc.), usually condensed on a window or in an optical cell at low 2 temperature, to preserve its structure for identification by spectroscopic or other means. 3

See [304]. 4 [3] 5

6 Mayr-Patz equation 7 Rate constants for the reactions of sp2-hybridized electrophiles with nucleophiles can 8 be expressed by the correlation 9 10

lg [k/ (dm3 mol-1 s-1)] = sN(E + N), 11 12 where E is the nucleophile-independent electrophilicity parameter, N is the electrophile-13 independent nucleophilicity parameter, and sN is the electrophile-independent 14 nucleophile-specific susceptibility parameter. 15

Note 1: This equation is equivalent to the conventional linear Gibbs energy 16 relationship lg k = Nu + sNE, where Nu = sNN. The use of N is preferred, because it 17 provides an approximate ranking of relative reactivities of nucleophiles. 18

Note 2: The correlation should not be applied to reactions with bulky electrophiles, 19 where steric effects cannot be neglected. Because of the way of parametrization, the 20 correlation is applicable only if one or both reaction centres are carbon. 21

Note 3: As the E parameters of the reference electrophiles are defined as solvent-22 independent, all solvent effects are shifted into the parameters N and sN. 23

Note 4: The equation transforms into the Ritchie equation for sN = 1. 24 Note 5: Applications to SN2-type reactions are possible if an electrophile-specific 25

susceptibility parameter is introduced. 26 See [305,306,307,308]. 27 See also Ritchie equation, Swain-Scott equation. 28 The Mayr scale is available at http://www.cup.lmu.de/oc/mayr/DBintro.htm 29

30 mechanism 31 Detailed description of the process leading from the reactants to the products of a 32 reaction, including a characterization as complete as possible of the composition, 33 structure, energy and other properties of reaction intermediates, products, and 34 transition states. 35

Note 1: An acceptable mechanism of a specified reaction (and there may be a 36 number of such alternative mechanisms not excluded by the evidence) must be 37 consistent with the reaction stoichiometry, the rate law, and with all other available 38 experimental data, such as the stereochemical course of the reaction. 39

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Note 2: Inferences concerning the electronic motions that dynamically interconvert 1 successive species along the reaction path (as represented by curved arrows, for 2 example) are often included in the description of a mechanism. 3

rev[3] 4 5 mechanism-based inhibition (suicide inhibition) 6 Irreversible inhibition of an enzyme by formation of covalent bond(s) between the 7 enzyme and the inhibitor, which is a substrate analogue that is converted by the enzyme 8 into a species that reacts with the enzyme. 9

rev[3] 10 11 medium 12 Phase (and composition of the phase) in which chemical species and their reactions 13 are studied. 14

[3] 15 16 Meisenheimer adduct (Jackson-Meisenheimer adduct) 17 Lewis adduct formed in nucleophilic aromatic substitution from a nucleophile (Lewis 18 base) and an aromatic or heteroaromatic compound, 19

20 Note 1: In cases where the substrate lacks electron-withdrawing groups, and 21

depending also on the nucleophile, the Meisenheimer adduct is not a minimum on the 22 potential-energy surface but a transition state. 23

See [309,310,311,312]. 24 Note 2: Analogous cationic adducts, such as 25

26

27 28 which are intermediates in electrophilic aromatic substitution reactions, are instead 29 called Wheland intermediates or s-adducts (or the discouraged term s-complexes). 30

See [313,314]. 31 rev[3] 32

H NO2

+

O2NOEt

NO2

NO2

O2N NO2

NO2

EtO OEt –

+ EtO– K+ K+

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1 melting point (corrected/uncorrected) 2 Temperature at which liquid and solid phases coexist in equilibrium, as measured with 3 a thermometer whose reading was corrected (or not) for the emergent stem that is in 4 ambient air. 5

Note: In current usage the qualification often means that the thermometer was/(was 6 not) calibrated or that its accuracy was/(was not verified). This usage is inappropriate 7 and should be abandoned. 8

[3] 9 10 mesolytic cleavage 11 Cleavage of a bond in a radical ion whereby a radical and an ion are formed. The term 12 reflects the mechanistic duality of the process, which can be viewed as homolytic or 13 heterolytic depending on how the electrons are assigned to the fragments. 14

See [315,316]. 15 [3] 16

17 mesomerism, mesomeric structure 18 obsolete 19 Resonance, resonance form 20

rev[3] 21 22 mesophase 23 Phase of a liquid crystalline compound between the crystalline and the isotropic liquid 24 phase. 25

See [317]. 26 [3] 27

28 metastable (chemical species) 29 deprecated 30 Transient (chemical species). 31

[3] 32 33 metathesis 34 Process formally involving the redistribution of fragments between similar chemical 35 species so that the bonds to those fragments in the products are identical (or closely 36 similar) to those in the reactants. 37

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Example: 1

2 Note: The term has its origin in inorganic chemistry, with a different meaning, but that 3

older usage is not applicable in physical organic chemistry. 4 [3] 5

6 micellar catalysis 7 Acceleration of a chemical reaction in solution by the addition of a surfactant at a 8 concentration higher than its critical micelle concentration so that the reaction can 9 proceed in the environment of surfactant aggregates (micelles). 10

Note 1: Rate enhancements may be due to a higher concentration of the reactants 11 in that environment, or to a more favourable orientation and solvation of the species, or 12 to enhanced rate constants in the micellar pseudophase of the surfactant aggregate. 13

Note 2: Micelle formation can also lead to a decreased reaction rate. 14 See [318]. 15 See also catalyst. 16 [3] 17

18 micelle 19 Aggregate of 1- to 1000-nm diameter formed by surfactants in solution, which exists in 20 equilibrium with the molecules or ions from which it is formed. 21

See [135]. 22 See also inverted micelle. 23 rev[3] 24

25 Michaelis-Menten kinetics 26 Appearance of saturation behavior in the dependence of the initial rate of reaction v0 27 on the initial concentration [S]0 of a substrate when it is present in large excess over 28 the concentration of an enzyme or other catalyst (or reagent) E, following the equation, 29 30

v0 = Vmax [S]0 /(KM + [S]0), 31 32 where v0 is the observed initial rate, Vmax is its limiting value at substrate saturation (i.e., 33 when [S]0 >> KM, so that all enzyme is bound to substrate), and KM is the substrate 34 concentration at which v0 = Vmax/2. 35

Note 1: Empirical definition, applying to any reaction that follows an equation of this 36 general form. 37

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Note 2: Often only initial rates are measured, at low conversion, so that [S]0 can be 1 considered as time-independent but varied from run to run. 2

Note 3: The parameters Vmax and KM (the Michaelis constant) can be evaluated from 3 slope and intercept of a linear plot of 1/v0 against 1/[S]0 (Lineweaver-Burk plot) or from 4 slope and intercept of a linear plot of [S]0/v0 against [S]0 (Eadie-Hofstee plot), but a 5 nonlinear fit, which is readily performed with modern software, is preferable. 6

Note 4: This equation is also applicable to the condition where E is present in large 7 excess, in which case [E] appears in the equation instead of [S]0. 8

Note 5: The term has been used to describe reactions that proceed according to the 9 scheme 10

11 12 in which case KM = (k–1 + kcat)/k1 (Briggs-Haldane conditions). It has more usually been 13 applied only to the special case in which k–1 >> kcat and KM = k–1/k1; in this case KM is a 14 true dissociation constant (Michaelis-Menten conditions). 15

See [319,320]. 16 [3] 17

18 microscopic diffusion control (encounter control) 19 Observable consequence of the limitation that the rate of a bimolecular chemical 20 reaction in a homogeneous medium cannot exceed the rate of encounter of the reacting 21 molecular entities. 22

Note: The maximum rate constant is usually in the range 109 to 1010 dm3 mol–1 s–1 in 23 common solvents at room temperature. 24

See also mixing control. 25 rev[3] 26

27 microscopic reversibility, principle of 28 In a reversible reaction the mechanism in one direction is exactly the reverse of the 29 mechanism in the other direction. 30

See also chemical reaction, detailed balancing. 31 [3] 32

33 migration 34 Transfer (usually intramolecular) of an atom or group during the course of a molecular 35 rearrangement. 36

Example (of a hydrogen migration): 37 38

RCH2CH=CH2 → RCH=CHCH3 39

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rev[3] 1 2 migratory aptitude 3 Tendency of a group to participate in a molecular rearrangement, relative to that of 4 another group, often in the same molecule. 5

Example: In the Baeyer-Villiger rearrangement of PhCOCH3, via intermediate 6 PhC(OH)(OOCOAr)CH3, the major product is CH3COOPh, by phenyl migration, rather 7 than PhCOOCH3. 8

Note: In nucleophilic rearrangements (migration to an electron-deficient centre) the 9 migratory aptitude of a group is loosely related to its capacity to stabilize a partial 10 positive charge, but exceptions are known, and the position of hydrogen in the series is 11 often unpredictable. 12

rev[3] 13 14 migratory insertion 15 Reaction that involves the migration of a group to another position on a metal centre, 16 with insertion of that group into the bond between the metal and the group that is in that 17 other position. 18

Examples: 19 CO insertion 20

21

22 23

Ziegler-Natta reaction 24 25

26 27

Note: On repetition of the reaction the substituent alternates between the two 28 positions. 29

rev[3] 30 31 minimum structural change, principle of 32 Principle that a chemical reaction is expected to occur with a minimum of bond changes 33 or with a minimum redistribution of electrons (although more complex reaction 34 cascades are also possible). 35

Example: 36 37

OC R OC R RCO RCO LL

-LL

L

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1 2 where the transfer of Cl is accompanied by migration of only one carbon, rather than a 3 more extensive molecular rearrangement involving migration of three methyls. 4

See [286]. 5 See also least nuclear motion, principle of. 6 rev[3] 7

8 minimum-energy reaction path (MERP) 9 Path of steepest-descent from a saddle point on a potential-energy surface in each 10 direction towards adjacent energy minima; equivalent to the energetically easiest route 11 from reactants to products. 12

See intrinsic reaction coordinate. 13 See [8]. 14

15 mixing control 16 Experimental limitation of the rate of reaction in solution by the rate of mixing of 17 solutions of the two reactants. 18

Note 1: Mixing control can occur even when the reaction rate constant is several 19 orders of magnitude less than that for an encounter-controlled reaction. 20

Note 2: Analogous (and more important) effects of the limitation of reaction rates by 21 the rate of mixing are encountered in heterogeneous (solid/liquid, solid/gas, liquid/gas) 22 systems. 23

See [321]. 24 See also microscopic diffusion control, stopped flow. 25 [3] 26 27 28 29

Möbius aromaticity 30 Feature of a monocyclic array of p orbitals in which there is a single out-of-phase 31 overlap (or, more generally, an odd number of out-of-phase overlaps), whereby the 32 pattern of aromatic character is opposite to Hückel systems; with 4n p electrons it is 33 stabilized (aromatic), whereas with 4n + 2 it is destabilized (antiaromatic). 34

Note 1: The name is derived from the topological analogy of such an arrangement of 35 orbitals to a Möbius strip. 36

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Note 2: The concept has been applied to transition states of pericyclic reactions.Note 1 3: In the electronically excited state 4n + 2 Möbius p-electron systems are stabilized, 2 and 4n systems are destabilized. 3

Note 4: A few examples of ground-state Möbius p systems are known [322,323]. 4 See [324,325]. 5 See also aromatic, Hückel (4n + 2) rule. 6 rev[3] 7

8 molecular entity 9 Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical 10 ion, complex, conformer, etc., identifiable as a separately distinguishable entity. 11

Note 1: Molecular entity is used in this glossary as a general term for singular entities, 12 irrespective of their nature, while chemical species stands for sets or ensembles of 13 molecular entities. Note that the name of a substance may refer to the molecular entity 14 or to the chemical species, e.g., methane, may mean either a single molecule of CH4 15 (molecular entity) or an ensemble of such species, specified or not (chemical species), 16 participating in a reaction. 17

Note 2: The degree of precision necessary to describe a molecular entity depends 18 on the context. For example "hydrogen molecule" is an adequate definition of a certain 19 molecular entity for some purposes, whereas for others it may be necessary to 20 distinguish the electronic state and/or vibrational state and/or nuclear spin, etc. of the 21 molecule. 22

[3] 23 24 molecular formula 25 List of the elements in a chemical species or molecular entity, with subscripts indicating 26 how many atoms of each element are included. 27

Note: In organic chemistry C and H are listed first, then the other elements in 28 alphabetical order. 29

See [29]. 30 See also empirical formula. 31

32 33 molecular mechanics (MM) (empirical force-field calculation) 34 Computational method intended to give estimates of structures and energies for 35 molecules. 36

Note: Even though such calculations can be made with either classical or quantum 37 mechanics (or both), the term molecular mechanics is widely understood as a classical 38 mechanics method that does not explicitly describe the electronic structure of the 39 molecular entities. It is based on the assumption of preferred bond lengths and angles, 40

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deviations from which lead to strain, and the existence of torsional interactions and 1 attractive and repulsive van der Waals and Coulombic forces between non-bonded 2 atoms, all of which are parametrized to fit experimental properties such as energies or 3 structures. In contrast, in the quantum mechanical implementation no such 4 assumptions/parameters are needed. 5

See [8,326]. 6 rev[3] 7

8 molecular metal 9 Non-metallic material whose properties, such as conductivity, resemble those of metals, 10 usually following oxidative doping. 11

Example: polyacetylene following oxidative doping with iodine. 12 [3] 13

14 molecular orbital 15 One-electron wavefunction describing a single electron moving in the field provided by 16 the nuclei and all other electrons of a molecular entity of more than one atom. 17

Note 1: Molecular orbitals describing valence electrons are often delocalized over 18 several atoms of a molecule. They are conveniently expressed as linear combinations 19 of atomic orbitals. They can be described as two-centre, multi-centre, etc. in terms of 20 the number of nuclei (or "centres") encompassed. 21

Note 2: For molecules with a plane of symmetry, a molecular orbital can be classed 22 as sigma (s) or pi (p), depending on whether the orbital is symmetric or antisymmetric 23 with respect to reflection in that plane. 24

See atomic orbital, orbital. 25 See [8]. 26 rev[3] 27

28 molecular rearrangement 29 Reaction of a molecular entity that involves a change of connectivity. 30

Note 1: The simplest type of rearrangement is an intramolecular reaction in which 31 the product is isomeric with the reactant (intramolecular isomerization). An example is 32 the first step of the Claisen rearrangement. 33 34

35

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Note 2: The definition of molecular rearrangements includes reactions in which there 1 is a migration of an atom or bond (unexpected on the basis of the principle of minimum 2 structural change), as in the reaction 3 4

CH3CH2CH2Br + AgOAc → (CH3)2CHOAc + AgBr 5 6 where the rearrangement step can formally be represented as the "1,2-shift" of hydride 7 between adjacent carbon atoms in a carbocation 8 9

CH3CH2CH2+ → (CH3)2CH+ 10 11

Note 3: Such migrations also occur in radicals, e.g., 12 13

PhC(CH3)2–CH2· → ·C(CH3)2CH2Ph (neophyl rearrangement) 14 15

Note 4: The definition also includes reactions in which an entering group takes up a 16 different position from the leaving group, with accompanying bond migration, such as 17 in the "allylic rearrangement": 18 19

(CH3)2C=CHCH2Br + HO– → (CH3)2C(OH)CH=CH2 + Br– 20 21

Note 5: A distinction can be made between intramolecular rearrangements (or "true" 22 molecular rearrangements) and intermolecular rearrangements (or apparent 23 rearrangements). In the former case the atoms and groups that are common to a 24 reactant and a product never separate into independent fragments during the 25 rearrangement stage, whereas in an intermolecular rearrangement a migrating group 26 becomes completely free from the parent molecule and is re-attached to a different 27 position in a subsequent step, as in the Orton reaction: 28 29

C6H5N(Cl)COCH3 + HCl → C6H5NHCOCH3 + Cl2 30 → o- and p-ClC6H4NHCOCH3 + HCl 31

See [327]. 32 rev[3] 33

34 molecular recognition 35 Attraction between specific molecules through noncovalent interactions that often 36 exhibit electrostatic and stereochemical complementarity between the partners 37

Note: The partners are usually designated as host and guest, where the host 38 recognizes and binds the guest with high selectivity over other molecules of similar size 39 and shape. 40

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1 molecularity 2 Number of reactant molecular entities that are involved in the "microscopic chemical 3 event" constituting an elementary reaction. 4

Note 1: For reactions in solution this number is always taken to exclude molecular 5 entities that form part of the medium and which are involved solely by virtue of their 6 solvation of solutes. 7

Note 2: A reaction with a molecularity of one is called "unimolecular", one with a 8 molecularity of two "bimolecular", and of three "termolecular". 9

See also chemical reaction, order of reaction. 10 [3] 11

12 molecule 13 An electrically neutral entity consisting of more than one atom. 14

Note: Rigorously, a molecule must correspond to a depression on the potential-15 energy surface that is deep enough to confine at least one vibrational state. 16

See also molecular entity. 17 [3] 18

19 More O'Ferrall - Jencks diagram 20 Conceptual visualization of the potential-energy surface for a reacting system, as a 21 function of two coordinates, usually bond lengths or bond orders. 22

Note 1: The diagram is useful for analyzing structural effects on transition-state 23 geometry and energy. 24

Note 2: According to the Hammond postulate, stabilization of the products relative to 25 the reactants (an effect parallel to the minimum-energy reaction path, MERP) shifts the 26 transition state away from the product geometry, whereas destabilization of the 27 products shifts the transition state towards the product geometry. As first noted by 28 Thornton, stabilization of a structure located off the assumed MERP in a direction 29 perpendicular to it shifts the transition state toward that more stabilized geometry (a 30 perpendicular effect); destabilization shifts the transition state in the opposite direction. 31

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1

2 3

Figure. More O'Ferrall-Jencks diagram for β-elimination reaction, with reactants, 4 products, and possible intermediates at the four corners. 5 6

Case 1: In a concerted elimination the transition state (‡1) is a saddle point near the 7 centre of the diagram, and the assumed MERP follows close to the diagonal. 8

Case 2: If the carbanion intermediate is stabilized, the transition state shifts toward 9 that intermediate, to a transition state (‡2) in which the C–H bond is more extensively 10 broken and the C–X bond is more intact. Conversely, if the carbocation intermediate is 11 stabilized, the transition state shifts towards the top-left corner of the Figure, with less 12 C–H bond breaking and more C–X bond making 13

Case 3: If leaving group X– is stabilised, energy decreases along the dotted vertical 14 arrow. It follows that in the resulting transition state (‡3) the C–H bond is more intact but 15 there is little change in the C–X bond, the shift of the transition state is the resultant of 16 a parallel (Hammond) component away from the top-right corner and a perpendicular 17 (anti-Hammond) component towards the top-left corner, yielding a transition state (‡3) 18 with less C–H bond breaking but approximately no change in the extent of C–X bond 19 making. 20

See [229,328,329,330,331]. 21 See also Hammond Postulate, anti-Hammond effect. 22 rev[3] 23

24

X

HB– +

BH + + X–

BH +X

HB– + + X–

‡1

‡2

dC-H

dC-X

‡3

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Morse potential 1 V(r) 2 (unit J) 3 Empirical function relating the potential energy of a molecule to the interatomic distance 4 r accounting for the anharmonicity of bond stretching: 5 6

V(r) = De{1 – exp[–a(r–re)]}2 7 8 where De is the bond-dissociation energy, re is the equilibrium bond length, and a is a 9 parameter characteristic of a given molecule. 10

See [8]. 11 12 mu 13 µ 14 (1) Symbol used to designate (as a prefix) a ligand that bridges two or more atoms. 15

Note: If there are more than two atoms being bridged, μ carries a subscript to denote 16 the number of atoms bridged. 17 (2) Symbol used to designate dipole moment as well as many other terms in physics 18 and physical chemistry. 19 20 multi-centre bond 21 Bond in which an electron pair is shared among three or more atomic centres. 22

Note 1: This may be needed when the representation of a molecular entity solely by 23 localized two-electron two-centre bonds is unsatisfactory, or when there are not enough 24 electrons to allow one electron pair shared between two adjacent atoms. 25

Note 2: This is restricted to σ bonds and does not apply to species with delocalized 26 p electrons. 27

Examples include the three-centre bonds in diborane B2H6 and in bridged 28 carbocations. 29

rev[3] 30 31 multident 32 multidentate 33

See ambident. 34 [3] 35

36 nanomaterial 37 Substance whose particles are in the size range of 1 to 100 nm. 38

Note: This may have chemical properties different from those of the corresponding 39 bulk material. 40

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1 narcissistic reaction 2 Chemical reaction that can be described as the automerization or enantiomerization of 3 a reactant into its mirror image (regardless of whether the reactant is chiral). 4

Examples are cited under degenerate rearrangement and fluxional. 5 See [332]. 6 rev[3] 7

8 neighbouring-group participation 9 Direct interaction of the reaction centre (usually, but not necessarily, an incipient 10 carbenium-ion centre) with a lone pair of electrons of an atom or with the electrons of a 11 s or p bond contained within the parent molecule but not conjugated with the reaction 12 centre. 13

Example: 14

15 16

Note 1: A distinction is sometimes made between n, s, and p participation. 17 Note 2: The neighbouring group serves as a nucleophile, as in SN2 reactions, except 18

that the nucleophile is intramolecular, so that this step is unimolecular. 19 Note 3: A rate increase due to neighbouring-group participation is known as 20

anchimeric assistance. 21 Note 4: Synartetic acceleration is the name given to the special case of participation 22

by electrons on a substituent attached to a b-carbon, relative to the leaving group 23 attached to the a-carbon, as in the example above. This term is deprecated. 24

See also intramolecular catalysis, multi-centre bond. 25 rev[3] 26

27 NHOMO 28 Next-to-highest occupied molecular orbital. 29

See subjacent orbital. 30 rev[3] 31

32 NIH shift 33 Intramolecular hydrogen migration that can be observed in enzymatic and chemical 34 hydroxylations of aromatic rings, as evidenced by appropriate deuterium labelling, as 35 in 36 37

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1 2

Note 1: In enzymatic reactions the NIH shift is thought to derive from the 3 rearrangement of arene oxide intermediates, but other pathways have been suggested. 4

Note 2: NIH stands for National Institutes of Health, where the shift was discovered. 5 See [333]. 6 [3] 7

8 nitrene 9 Generic name for the species HN and substitution derivatives thereof, containing an 10 electrically neutral univalent nitrogen atom with four nonbonding electrons. 11

Note 1: Two nonbonding electrons may have antiparallel spins (singlet state) or 12 parallel spins (triplet state). 13

Note 2: The name is the strict analogue to carbene and, as a generic name, it is 14 preferred to a number of alternative proposed (imene, imine radical, aminylene, azene, 15 azylene, azacarbene, imin, imidogen). 16

See [97,334]. 17 [3] 18

19 no-bond resonance 20 double-bond-no-bond resonance 21 Inclusion of one or more contributing structures that lack the s bond of another 22 contributing structure. 23

Example: 24

25 See hyperconjugation. 26 rev[3] 27

28 nonclassical carbocation 29 Carbocation that has delocalized (bridged) bonding s electrons. 30

See [89,335]. 31 rev[3] 32

33

C CH

+H3CH3C H

HC C

H+

H3CH3C H

H

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non-Kekulé structure 1 Compound with unpaired electrons for which no Lewis structures are possible with all 2 bonding electrons paired in single or double bonds. 3

Examples: 4 5

6 7

Note: For the second example ("trimethylenemethane") the isomer shown (valence 8 tautomer methylidenecyclopropane) is a Kekulé structure. 9

See [336]. 10 11 nucleofuge 12 Leaving group that carries away the bonding electron pair in a nucleophilic substitution 13 reaction. 14

Example: In the hydrolysis of a chloroalkane, Cl– is the nucleofuge. 15 16

R–Cl + H2O → ROH + HCl 17 18

Note 1: Nucleofugality, commonly called leaving-group ability, characterizes the 19 relative rates of atoms or groups to depart with the bonding electron pair from a 20 reference substrate. Nucleofugality depends on the nature of the reference reaction and 21 is not the reverse of nucleophilicity. 22

Note 2: Protofugality is a special type of nucleofugality, characterizing the relative 23 rates of proton transfer from a series of Brønsted acids H–X to a common Brønsted 24 base. 25

See [194]. 26 See also electrofuge, nucleophile. 27 rev[3] 28

29 nucleophile (n.), nucleophilic (adj.) 30 Reactant that forms a bond to its reaction partner (the electrophile) by donating both of 31 its bonding electrons. 32

Note 1: A "nucleophilic substitution reaction" is a heterolytic reaction in which the 33 reagent supplying the entering group acts as a nucleophile. 34

Example: 35 36

MeO– (nucleophile) + EtCl → MeOEt + Cl– (nucleofuge) 37 38

CH2CH2

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Note 2: Nucleophilic reagents are Lewis bases. 1 Note 3: The term "nucleophilic" is also used to designate the apparent polar character 2

of certain radicals, as inferred from their higher relative reactivity with reaction sites of 3 lower electron density. 4

See also nucleophilicity, order of reaction. 5 [3] 6

7 nucleophilic catalysis 8 Catalysis by a Lewis base, involving conversion of a substrate with low electrophilicity 9 into an intermediate with higher electrophilicity. 10

Example 1: hydrolysis of acetic anhydride in aqueous solution, catalysed by 4-11 (dimethylamino)pyridine 12

Me2NC5H4N + (CH3CO)2O → (Me2NC5H4NCOCH3)+ + CH3CO2– 13 then (Me2NC5H4COCH3)+ + H2O → Me2NC5H4N + CH3CO2H + H+aq 14 15

Example 2: SN2 reaction of CH3CH2Cl with HO–, catalyzed by I–: 16 17

CH3CH2Cl + I– → Cl– + CH3CH2I, then CH3CH2I + HO– → CH3CH2OH + I– 18 19

See also nucleophilicity. 20 rev[3] 21

22 nucleophilic substitution 23 Heterolytic reaction in which an entering group adds to the electrophilic part of the 24 substrate and in which the leaving group, or nucleofuge, retains both electrons of the 25 bond that is broken, whereupon it becomes another potential nucleophile. 26

Example: 27 28

CH3Br (substrate) + HO– (nucleophile) → CH3OH + Br– (nucleofuge) 29 30

Note 1: It is arbitrary to emphasize the nucleophile and ignore the feature that this is 31 also an electrophilic substitution, but the distinction depends on the nucleophilic nature 32 of the reactant that is considered to react with the substrate. 33

Note 2: Nucleophilic substitution reactions are designated as SN1 or SN2, depending 34 on whether they are unimolecular or bimolecular, respectively. Mechanistically, these 35 correspond to two-step and one-step processes, respectively. SN1 reactions follow first-36 order kinetics but SN2 reactions do not always follow second-order kinetics. 37

See order of reaction, rate coefficient. 38 39 40

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nucleophilicity 1 Relative reactivity of a nucleophile toward a common electrophile. 2

Note 1: The concept is related to Lewis basicity. However, whereas the Lewis 3 basicity of a base B: is measured by its equilibrium constant for adduct formation with 4 a common acid A, the nucleophilicity of a Lewis base B: is measured by the rate 5 coefficient for reaction with a common substrate A–Z, often involving formation of a 6 bond to carbon. 7

Note 2: Protophilicity is a special case of nucleophilicity, describing the relative rates 8 of reactions of a series of Lewis bases B: with a common Brønsted acid H–Z. The term 9 “protophilicity” is preferred over the alternative term “kinetic basicity” because basicity 10 refers to equilibrium constants, whereas “philicity” (like “fugality”) refers to rate 11 constants. 12

See [192,193,307]. 13 See also Brønsted basicity, electrophilicity, Lewis basicity, Mayr-Patz equation, 14

Ritchie equation, Swain-Scott equation. 15 rev[3] 16

17 n-s* delocalization (n-s* no-bond resonance) 18 Delocalization of a lone pair (n) into an antibonding s-orbital (s*). 19

See also anomeric effect, hyperconjugation, resonance. 20 [3] 21

22 octanol-water partition ratio (KOW): 23 Equilibrium concentration of a substance in octan-1-ol divided by its equilibrium 24 concentration in water. 25

Note: This is a measure of the lipophilicity of a substance. It is used in 26 pharmacological studies and in the assessment of environmental fate and transport of 27 organic chemicals. 28

See [337,338]. 29 See also partition ratio. 30

31 octet rule 32 Electron-counting rule that the number of lone-pair electrons on a first-row atom plus 33 the number of electron pairs in that atom's bonds should be 8. 34 35 onium ion 36 (1) Cation derived by addition of a hydron to a mononuclear parent hydride of the 37 nitrogen, chalcogen, and halogen family, e.g., H4N+ ammonium ion. 38 (2) Derivative formed by substitution of the above parent ions by univalent groups, e.g., 39 (CH3)2SH+ dimethylsulfonium (dimethylsulfanium), (CH3CH2)4N+ tetraethylammonium. 40

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See [97]. 1 See also carbenium ion, carbonium ion. 2 rev[3] 3

4 optical yield 5 Ratio of the optical purity of the product to that of the chiral precursor or reactant. 6

Note 1: This should not be confused with enantiomeric excess. 7 Note 2: The optical yield is not related to the chemical yield of the reaction. 8 See [11]. 9 See stereoselectivity. 10 [3] 11

12 orbital (atomic or molecular) 13 Wavefunction depending on the spatial coordinates of only one electron. 14

Note: An orbital is often illustrated by sketching contours, often very approximate, on 15 which the wavefunction has a constant value or by indicating schematically the 16 envelope of the region of space in which there is an arbitrarily fixed high probability (say 17 95 %) of finding the electron occupying that region, and affixing also the algebraic sign 18 (+ or –) of the wavefunction in each part of that region, or suggesting the sign by 19 shading. 20

Examples: 21 22

23 24

See atomic orbital, molecular orbital. 25 See [8]. 26 rev[3] 27

28 orbital steering 29 Concept expressing the principle that the energetically favourable stereochemistry of 30 approach of two reacting species is governed by the most favourable overlap of their 31 appropriate orbitals. 32

rev[3] 33 34 orbital symmetry 35 Behaviour of an atomic orbital or molecular orbital under molecular symmetry 36 operations, such that under reflection in a symmetry plane or rotation by 180º around a 37

HH

HH H

H

1s 2pz sp3 hybrid bonding MOH

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symmetry axis the phase of the orbital is either unchanged (symmetric) or changes sign 1 (antisymmetric), whereby positive and negative lobes are interchanged. 2

Examples: 3 (1) The orbital of an idealized single bond is s, with cylindrical symmetry. 4 (2) A p-orbital or p-bond orbital has p symmetry, i.e., it is antisymmetric with respect 5

to reflection in a plane passing through the atomic centres with which it is associated. 6 (3) The HOMO of 1,3-butadiene (illustrated below by its component atomic orbitals) 7

is antisymmetric with respect to 180º rotation about an axis through the C2–C3 bond 8 and perpendicular to the molecular plane. 9 10 11 12 13 14 15

See [107,123]. 16 See also conservation of orbital symmetry, sigma, pi. 17 rev[3] 18

19 order of reaction 20 Exponent a, independent of concentration and time, in the differential rate equation 21 (rate law) relating the macroscopic (observed, empirical, or phenomenological) rate of 22 reaction v to cA the concentration of one of the chemical species present, as defined by 23 24

𝛼 = $𝜕 ln{𝑣}𝜕 ln{c!}

+[#]

,…

25

The argument in the ln function should be of dimension 1. Thus, reduced quantities 26 should be used, i.e., the quantity divided by its unit, {v} = v/(mol dm-3 s-1) and {𝑐%} = 27 cA/(mol dm-3). 28

Note 1: A rate equation can often be expressed in the form v = k [A]a[B]b... , 29 describing the dependence of the rate of reaction on the concentrations [A], [B],..., 30 where exponents a, b, ... are independent of concentration and time and k is 31 independent of [A], [B], ... . In this case the reaction is said to be of order a with respect 32 to A, of order b with respect to B,..., and of (total or overall) order n = a + b +... . The 33 exponents a, b,... sometimes called "partial orders of reaction", can be positive or 34 negative, integral, or rational nonintegral numbers. 35

Note 2: For an elementary reaction a partial order of reaction is the same as the 36 stoichiometric number. The overall order is then the same as the molecularity. For 37 stepwise reactions there is no general connection between stoichiometric numbers and 38

HH

HH H

H

H

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partial orders. Such reactions may have more complex rate laws, so that an apparent 1 order of reaction may vary with the concentrations of the chemical species involved and 2 with the progress of the reaction: in such cases it is not useful to speak of orders of 3 reaction, although apparent orders of reaction may be deducible from initial rates. 4

Note 3: In a stepwise reaction, orders of reaction may in principle be assigned to the 5 elementary steps. 6

Note 4: For chemical rate processes occurring in systems for which concentration 7

changes are not measurable, as in the case of a dynamic equilibrium aA ,!pP, and if a 8

chemical flux f–A is found experimentally (e.g., by NMR line-shape analysis) to be 9 related to the concentration of A and to concentrations of other species B, ..., by the 10 equation 11 12

f–A = k[A]a[B]b ... 13 14 then the reaction is of order α with respect to A... and of total (or overall) order 15 (=a + b +...). 16 Note 5: If the overall rate of reaction is given by 17 18

v = k[A]a[B]b 19 20 but [B] remains constant in any particular sample (but can vary from sample to sample), 21 then the order of the reaction in A will bea, and the rate of disappearance of A can be 22 expressed in the form 23 24

vA = kobs[A]a 25 26 The proportionality factor kobs is called the "observed rate coefficient" and is related to 27 the rate constant k by 28 29

kobs = k[B]b 30 31

Note 6: For the frequent case where a = 1, kobs is often referred to as a "pseudo-first-32 order rate coefficient". 33

See [13]. 34 See also kinetic equivalence, rate coefficient, rate constant. 35 rev[3] 36

37 38 39

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organocatalysis 1 Catalysis by small organic molecules, as distinguished from catalysis by (transition) 2 metals or enzymes. 3

Note 1: Frequently used organocatalysts are secondary amines (covalent catalysis 4 by generation of enamines or of iminium ions as reactive intermediates) and thioureas 5 (hydrogen-bonding catalysis). 6

Note 2: Organocatalysts are often employed for enantioselectivity. 7 Note 3: The mechanisms employed by organocatalysts are examples of general acid 8

catalysis, general base catalysis, nucleophilic catalysis, specific acid catalysis, specific 9 base catalysis. 10

See [339,340,341,342]. 11 12 outer-sphere electron transfer 13 Feature of an electron transfer between redox centres not sharing a common atom or 14 group. 15

Example: 16 MnO4– + Mn*O42– ⇄ MnO42– + Mn*O4– 17 Note 1: In the transition state the interaction between the relevant electronic orbitals 18

of the two centres is weak (below 20 kJ mol–1), and the electron(s) must tunnel through 19 space. 20

Note 2: If instead the donor and the acceptor exhibit a strong electronic coupling, 21 often through a ligand that bridges both, the reaction is described as inner-sphere 22 electron transfer. 23

Note 3: These two terms derive from studies of metal complexes, and for organic 24 reactions the terms "nonbonded' and 'bonded" electron transfer are often used. 25

See [9,182,343]. 26 rev[3] 27

28 oxidation 29 (1) Removal of one or more electrons from a molecular entity. 30 (2) Increase in the oxidation number of an atom within a substrate, see [344]. 31 (3) Gain of oxygen and/or loss of hydrogen by an organic substrate. 32

Note 1: All oxidations meet criterion (2) and many meet criterion (3), but this is not 33 always easy to demonstrate. Alternatively, an oxidation can be described as the 34 transformation of an organic substrate by removal of one or more electrons from the 35 substrate, often accompanied by gain or loss of water, hydrons, and/or hydroxide, or by 36 nucleophilic substitution, or by molecular rearrangement. 37

Note 2: This formal definition allows the original idea of oxidation (combination with 38 oxygen), together with its extension to removal of hydrogen, as well as processes 39 closely akin to this type of transformation (and generally regarded in organic chemistry 40

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to be oxidations and to be effected by "oxidizing agents") to be descriptively related to 1 definition (1). For example the oxidation of methane to chloromethane may be 2 considered as 3

CH4 – 2e– – H+ + HO– = CH3OH (oxidation), 4 followed by 5

CH3OH + HCl = CH3Cl + H2O (neither oxidation nor reduction) 6 rev[3] 7

8 oxidation number 9 oxidation state 10 Number assigned to a carbon atom in a covalent organic compound according to 11 12

NOx = NX + NO + NN – NH – NM 13 14 where NX, NO, NN, NH, and NM are the numbers of bonds to halogen, oxygen (or sulfur), 15 nitrogen, hydrogen, and a metal, respectively. 16

Examples: 17 18

CH3MgBr (-4), CH3CH3 (-3), CH2=CH2 (-2), CH3OH (-2), CH2Cl2 (0), 19 CH3CH=O (+1), HCN (+2), CCl4 (+4). 20

21 Note 1: This assignment is based on the convention that each attached atom more 22

electronegative than carbon contributes +1, while each atom less electronegative 23 (including H) contributes -1, and an attached carbon contributes zero. 24

Note 2: Oxidation numbers are not significant in themselves, but changes in oxidation 25 number are useful for recognizing whether a reaction is an oxidation or a reduction or 26 neither. 27

Note 3: A different system is used for transition-metal species [345]. 28 See also electronegativity, oxidation. 29 rev[3] 30 31

oxidative addition 32 Insertion of the metal of a metal complex into a covalent bond involving formally an 33 overall two-electron loss on one metal or a one-electron loss on each of two metals, 34 i.e., 35

LnMm!-!./!→!0nMm+2(X)(Y) 36

or 2 LnMm!-!./!→!0nMm+1(X) + LnMm+1(Y) 37

rev[3] 38 39

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oxidative coupling 1 Coupling of two molecular entities through an oxidative process, usually catalysed by a 2 transition-metal compound. 3

Example (where the oxidant can be O2 or Cu(II) or others): 4 5

6 rev[3] 7

8 parallel effect 9 Change of the position of the transition state upon stabilization or destabilization of a 10 structure (or structures) along the assumed minimum-energy reaction path. 11

See Hammond postulate, More O'Ferrall - Jencks diagram. 12 13 parallel reaction 14

See composite reaction. 15 [3] 16

17 paramagnetism 18 Property of substances having a magnetic susceptibility greater than 0, whereby they 19 are drawn into a magnetic field. 20

See also diamagnetism. 21 [3] 22

23 partial rate factor pfZ, mfZ 24 Rate constant for substitution at one specific site in an aromatic compound divided by 25 the rate constant for substitution at one position in benzene. 26

Note 1: The partial rate factor pfZ for para-substitution in a monosubstituted benzene 27 C6H5Z is related to the rate constants k(C6H5Z) and k(C6H6) for the total reactions (i.e., 28 at all positions) of C6H5Z and benzene, respectively, and fpara (the fraction of para-29 substitution in the total product formed from C6H5Z, usually expressed as a percentage) 30 by the relation 31 32

𝑝&' = 6𝑘(C(H)Z)𝑘(C(H()

𝑓*+,+ 33

34 Similarly for meta-substitution: 35 36

𝑚&' =

6𝑘(C(H)Z)2𝑘(C(H()

𝑓-./+ 37

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1 The symbols fpZ, fmZ, foZ are also in use. 2

Note 2: The term applies also to the ipso position, and it can be extended to other 3 substituted substrates undergoing parallel reactions at different sites with the same 4 reagent according to the same rate law. 5

See [262,346,347]. 6 See also selectivity. 7 rev[3] 8

9 partition ratio (partition constant, distribution ratio) P 10 Concentration of a substance in one phase divided by its concentration in another 11 phase, at equilibrium. 12

Example, for an aqueous/organic system the partition ratio (or distribution ratio D) is 13 given by 14

P = corg(A)/caq(A) 15 Note 1: The most common way of applying P in correlation analysis or quantitative 16

structure-activity relationships is as lg P. 17 Note 2: The parameter P is extensively used as an indicator of the capacity of a 18

molecular entity to cross biological membranes by passive diffusion. 19 Note 3: The term partition coefficient is in common usage in toxicology but is not 20

recommended for use in chemistry and should not be used as a synonym for partition 21 constant, partition ratio, or distribution ratio. 22

See [167,232]. 23 See also Hansch constant, octanol-water partition ratio. 24

25 pericyclic reaction 26 Chemical reaction in which concerted reorganization of bonding takes place throughout 27 a cyclic array of continuously bonded atoms. 28

Note 1: It may be viewed as a reaction proceeding through a fully conjugated cyclic 29 transition state. 30

Note 2: The term embraces a variety of processes, including cycloaddition, 31 cheletropic reaction, electrocyclic reaction and sigmatropic rearrangement, etc. 32 (provided they are concerted). 33

See also pseudopericyclic. 34 [3] 35

36 permittivity, relative εr 37 dielectric constant (obsolete) 38

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Measure of the reduction of the magnitude of the potential energy of interaction between 1 two charges on going from vacuum to a condensed medium, expressed as the ratio of 2 the former to the latter. 3

Note: The term dielectric constant is obsolete. Moreover, the dielectric constant is 4 not a constant since it depends on frequency. 5

See [12]. 6 7 perpendicular effect 8 Change of the position of the transition state upon stabilization or destabilization of a 9 structure (or structures) that lies off the assumed minimum-energy reaction path. 10

See anti-Hammond effect, Hammond postulate, More O'Ferrall - Jencks diagram. 11 rev[3] 12

13 persistence 14 Characteristic of a molecular entity that has an appreciable lifetime (minutes or 15 nanoseconds or other, depending on context). 16

Note 1: Dilute solution or inert solvent may be required for persistence. 17 Note 2: Persistence is a kinetic or reactivity property, whereas, in contrast, stability 18

(being stable) is a thermodynamic property. 19 See [348]. 20 See also transient. 21 rev[3] 22

23 pH-rate profile 24 Plot of observed rate coefficient, or more usually the decadic logarithm of its numerical 25 value, against solution pH, other variables being kept constant. 26

[3] 27 28 phase-transfer catalysis 29 Enhancement of the rate of a reaction between chemical species located in different 30 phases (immiscible liquids or solid and liquid) by addition of a small quantity of an agent 31 (called the phase-transfer catalyst) that extracts one of the reactants, most commonly 32 an anion, into the other phase so that reaction can proceed. 33

Note 1: These catalysts are often onium ions (e.g., tetraalkylammonium ions) or 34 complexes of inorganic cations (e.g., as crown ether complexes). 35

Note 2: The catalyst cation is not consumed in the reaction although an anion 36 exchange does occur. 37

Example: 38 CH3(CH2)7Cl in decane + aqueous Na+CN– + catalytic PhCH2N(CH2CH3)3+ 39 = CH3(CH2)7CN + aqueous Na+Cl– 40

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rev[3] 1 2 phenonium ion 3

See bridged carbocation. 4 [3] 5

6 photochromism 7 Reversible transformation of a molecular entity between two forms, A and B, having 8 different absorption spectra, induced in one or both directions by absorption of 9 electromagnetic radiation. 10

Note 1: The thermodynamically stable form A is transformed by irradiation into form 11 B. The back reaction can occur thermally (photochromism of type T) or photochemically 12 (photochromism of type P). 13

Note 2: The spectral change is typically, but not necessarily, of visible colour. 14 Note 3: An important parameter is the number of cycles that a photochromic system 15

can undergo. 16 See [9]. 17 18

photolysis 19 Bond cleavage induced by ultraviolet, visible, or infrared radiation. 20

Example: 21 Cl2 → 2 Cl• 22

Note: The term is used incorrectly to describe irradiation of a sample without any 23 bond cleavage, although in the term “flash photolysis” this usage is accepted. 24

See [9]. 25 [3] 26

27 photostationary state 28 Steady state reached by a chemical system undergoing photochemical reaction, such 29 that the rates of formation and disappearance are equal for each of the transient 30 molecular entities formed. 31

See [9]. 32 33 pi-adduct 34

See p-adduct. 35 36 pi-bond 37

See s, p. 38 39 40

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polar aprotic solvent 1 See dipolar non-HBD solvent. 2 [3] 3

4 polar effect 5 All the interactions whereby a substituent on a reactant molecule RY modifies the 6 electrostatic forces operating at the reaction centre Y, relative to the reference standard 7 RoY. 8

Note 1: These forces may be governed by charge separations arising from 9 differences in the electronegativity of atoms (leading to the presence of dipoles), by the 10 presence of monopoles, or by electron delocalization. 11

Note 2: It is distinguished from a steric effect. 12 Note 3: Sometimes, however, the term polar effect is taken to refer to the influence, 13

other than steric, that non-conjugated substituents exert on reaction rates or equilibria, 14 thus excluding effects of electron delocalization between a substituent and the 15 molecular framework to which it is attached. 16

See also electronic effects (of substituents), field effect, inductive effect. 17 [3] 18

19 polar solvent 20 Liquid composed of molecules with a significant dipole moment, capable of dissolving 21 ions or other molecules with significant dipole moments. 22

See polarity. 23 rev[3] 24

25 polarity (of a bond) 26 Characteristic of a bond between atoms of different electronegativity, such that the 27 electrons in that bond are not shared equally. 28 29 polarity (of a solvent) 30 Overall solvation capability (solvation power) of a solvent toward solutes, which 31 depends on the action of all possible intermolecular interactions between solute ions or 32 molecules and solvent molecules, excluding interactions leading to definite chemical 33 alterations of the ions or molecules of the solute. 34

Note: Quantitative measures of solvent polarity include relative permittivity (dielectric 35 constant) and various spectroscopic parameters. 36


40

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polarizability 1 a 2 (SI unit: C m2 V–1) 3 electric polarizability 4 Induced dipole moment, µinduced, divided by applied electric field strength E 5

a!'!!µinduced1E 6

Note 1: The polarizability represents the ease of distortion of the electron cloud of a 7 molecular entity by an electric field (such as that due to the proximity of a charged 8 species). 9

Note 2: Polarizability is more often expressed as polarizability volume, with unit cm3, 10 where e0 is the permittivity of vacuum: 11 12

a /cm3""#"" :;,

<π>-!"𝝁"@

13

14 Note 3: In general, polarizability is a tensor that depends on direction, for example, 15

depending on whether the electric field is along a bond or perpendicular to it, and the 16 induced dipole may not even be along the direction of the electric field. However, in 17 ordinary usage the term refers to the mean polarizability, the average over three 18 rectilinear axes of the molecule. 19

rev[3] 20 21 polydent 22 polydentate 23

See ambident. 24 [3] 25

26 potential-energy profile 27 Curve describing the variation of the potential energy of a system of atoms as a function 28 of a single coordinate. 29

Note 1: For an elementary reaction the relevant coordinate is the reaction coordinate, 30 which is a measure of progress along the minimum-energy reaction path (MERP) from 31 a saddle point on a potential-energy surface in each direction toward adjacent energy 32 minima. For a stepwise reaction it is the succession of reaction coordinates for the 33 successive individual reaction steps. For a reaction involving a bifurcation each branch 34 requires a different reaction coordinate and has its own profile. 35

Note 2: A profile constructed as a function of an arbitrary internal coordinate (for 36 example, a bond distance) is not guaranteed to pass through a saddle point on the 37 corresponding potential-energy surface; in order to do so, it must be smooth, 38

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continuous, and follow the path of lowest energy connecting the reactant and product 1 energy minima. 2

Examples: (one-step reaction, two-step reaction) 3 4

5 6

See [349]. 7 See also Gibbs energy diagram, potential-energy surface. 8 rev[3] 9

10 potential-energy surface (PES) 11 Surface describing the variation within the Born-Oppenheimer approximation of the 12 potential energy of a system of atoms as a function of a set of internal coordinates. 13

Note 1: A minimum on a PES is characterized by positive curvature in all directions 14 and corresponds to a structure that is stable with respect to small displacements away 15 from its equilibrium geometry; for this structure (a reactant, intermediate or product) all 16 vibrational frequencies are real. A saddle point is characterized by positive curvature in 17 all directions except for one with negative curvature and corresponds to a transition 18 structure, for which one vibrational frequency is imaginary. A local maximum is 19 characterized by negative curvatures in two (or more) directions and has two (or more) 20 imaginary frequencies; it is sometimes called a second-order (or higher-order) saddle 21 point. 22

Note 2: It is usual to select only two coordinates in order to represent the surface, 23 with potential energy as the third dimension, or alternatively as a two-dimensional 24 contour map. For example, a PES for a simple reacting triatomic system A–B + C!!→!!25 A + B–C, could be constructed using the A···B and B···C distances as two internal 26 coordinates; the third independent coordinate could be the ABC angle or the A···C 27 distance, and its value could either be kept fixed or else be relaxed to minimize the 28 energy at each point on the (A···B, B···C) surface. 29

Note 3: The path of steepest-descent from a saddle point in each direction towards 30 adjacent energy minima defines a minimum-energy reaction path (MERP) that is 31 equivalent to the energetically easiest route from reactants to products. The change in 32 potential energy along this path across the PES defines a potential-energy profile for 33

E

reaction coordinate

‡

reaction coordinate

‡1‡2

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the elementary reaction. Progress along this path is measured by the value of the 1 reaction coordinate. 2

Note 4: In general there is neither a unique set of internal coordinates nor a unique 3 choice of two coordinates with which to construct a PES of reduced dimensionality. 4 Consequently there is no guarantee of a smooth and continuous PES containing a 5 saddle point connecting reactant and product minima. 6

See [8,73,350,351]. 7 See: bifurcation, minimum-energy reaction path, potential-energy profile, reaction 8

coordinate, transition structure. 9 rev[3] 10

11 potential of mean force (PMF) 12 Free energy as a function of a set of coordinates, the negative gradient of which gives 13 the average force acting on that configuration averaged over all other coordinates and 14 momenta within a statistical distribution. If the averaging is performed within a canonical 15 ensemble (constant volume, temperature, and number of particles) the PMF is 16 equivalent to the Helmholtz energy, but if it is performed within an isobaric-isothermal 17 ensemble (constant pressure, temperature, and number of particles) the PMF is 18 equivalent to the Gibbs energy. 19

Note 1: Commonly, the PMF acting upon a selected geometric variable and averaged 20 over the coordinates and momenta of all other geometric variables is evaluated for a 21 succession of constrained values of the selected variable, thereby generating 22 (generically) a free-energy profile with respect to the selected reaction coordinate (e.g., 23 a bond distance or angle, or a combination of internal coordinates); specifically this is 24 either a Helmholtz-energy or a Gibbs-energy profile, depending upon the choice of 25 ensemble for the statistical averaging within a computational simulation. 26

Note 2: Selection of two geometric variables as reaction coordinates allows a free-27 energy surface to be computed as a two-dimensional PMF. 28

Note 3: Molecular simulations often yield Helmholtz energies, not Gibbs energies, 29 but for condensed phases the difference is usually neglected. 30

See: free energy, reaction coordinate. 31 32 pre-association 33 Step on the reaction path of some stepwise reactions in which the molecular entity C 34 forms an encounter pair or encounter complex with A prior to the reaction of A to form 35 product. 36

Example: 37

38

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1 Note 1: In this mechanism the chemical species C may but does not necessarily 2

assist the formation of B from A, which may itself be a bimolecular reaction with some 3 other reagent. 4

Specific example (aminolysis of phenyl acetate): 5 6

7 8

Note 2: Pre-association is important when B is too short-lived to permit B and C to 9 come together by diffusion. In the specific example, T± would dissociate faster than 10 general acid HA can diffuse to it. Experimentally, the Brønsted α is > 0, which is 11 inconsistent with rate-limiting diffusion and hydron transfer. 12

See [352]. 13 See also Brønsted relation, microscopic diffusion control, spectator mechanism. 14 rev[3] 15

16 precursor complex 17

See encounter complex. 18 [3] 19

20 pre-equilibrium 21 prior equilibrium 22 Rapid reversible step preceding the rate-limiting step in a stepwise reaction. 23

Example: 24

25 26

See also kinetic equivalence, steady state. 27

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[3] 1 2 pre-exponential factor 3

See energy of activation, entropy of activation. 4 [3] 5

6 principle of nonperfect synchronization 7 Consideration applicable to reactions in which there is a lack of synchronization 8 between bond formation or bond rupture and other changes that affect the stability of 9 products and reactants, such as resonance, solvation, electrostatic, hydrogen bonding 10 and polarizability effects. 11

Note: The principle states that a product-stabilizing factor whose development lags 12 behind bond changes at the transition state, or a reactant-stabilizing factor whose loss 13 is ahead of bond changes at the transition state, increases the intrinsic barrier and 14 decreases the rate constant of a reaction. For a product-stabilizing factor whose 15 development is ahead of bond changes, or a reactant-stabilizing factor whose loss lags 16 behind bond changes, the opposite relations hold. The reverse effects are observable 17 for factors that destabilize a reactant or product. 18

See [108]. 19 See also imbalance, synchronous. 20

21 prior equilibrium 22

See pre-equilibrium. 23 [3] 24

25 product-determining step 26 Step of a stepwise reaction in which the product distribution is determined. 27

Note: The product-determining step may be identical to, or may occur later than, the 28 rate-determining step in the reaction. 29

[3] 30 31 product-development control 32 Case of kinetic control in which the selectivity of a reaction parallels the relative 33 (thermodynamic) stabilities of the products. 34

Note: Product-development control arises because whatever effect stabilizes or 35 destabilizes a product is already operative at the transition state. Therefore it is usually 36 associated with a transition state occurring late on the minimum-energy reaction path. 37

See also steric-approach control, thermodynamic control. 38 [3] 39

40

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promotion 1 See pseudo-catalysis. 2 [3] 3

4 propagation 5

See chain reaction. 6 [3] 7

8 propargylic substitution 9

See allylic substitution reaction. 10 11 protic 12

See protogenic. 13 [3] 14

15 protic solvent 16 Solvent that is capable of acting as a hydrogen-bond donor. 17

See HBD solvent. 18 19 protofugality 20 special case of nucleofugality, describing the relative rates of transfer of a proton (more 21 generally: hydron) from a series of Brønsted acids H–X to a common Brønsted base. 22

Note: This term has the advantage over the commonly used “kinetic acidity” that 23 philicity and fugality are associated with kinetics, while acidity and basicity are 24 associated with thermodynamics. 25

See [194]. 26 See also Brønsted acidity, nucleofugality, protophilicity. 27 28

protogenic (solvent) 29 HBD (hydrogen bond donor) solvent. 30 Capable of acting as a proton (hydron) donor. 31

Note 1: Such a solvent may be a strong or weak Brønsted acid. 32 Note 2: The term is preferred to the synonym protic or to the more ambiguous 33

expression acidic. 34 See protophilic solvent. 35 [3] 36

37 protolysis 38 proton (hydron)-transfer reaction. 39

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Note: Because of its misleading similarity to hydrolysis, photolysis, etc., this use is 1 discouraged. 2

See also autoprotolysis. 3 rev[3] 4

5 proton affinity 6 Negative of the enthalpy change in the gas phase reaction between a proton (more 7 appropriately hydron) and the chemical species concerned, usually an electrically 8 neutral or anionic species, to give the conjugate acid of that species. 9

Note 1: For an anion A–, the proton affinity is the negative of the enthalpy of the 10 heterolytic dissociation (in the gas phase) of the Brønsted acid HA. 11

Note 2: Proton affinity is often, but unofficially, abbreviated as PA. 12 Note 3: Affinity properly refers to Gibbs energy. 13 See also gas phase basicity, gas phase acidity. 14 See [2]. See also [213,353]. 15 rev[3] 16

17 protonation 18 Attachment of the ion 1H+ (of relative atomic mass » 1). 19

See also hydronation. 20 21 proton–transfer reaction 22 Chemical reaction, the main feature of which is the intermolecular or intramolecular 23 transfer of a proton (hydron) from one binding site to another. 24

Example: 25 CH3COOH + (CH3)2C=O → CH3CO2– + (CH3)2C=OH+ 26

Note: In the detailed description of proton-transfer reactions, especially of rapid 27 proton transfers between electronegative atoms, it should always be specified whether 28 the term is used to refer to the overall process (including the more-or-less encounter-29 controlled formation of a hydrogen-bonded complex and the separation of the products) 30 or just to the proton-transfer event (including solvent rearrangement) by itself. 31

See also autoprotolysis, microscopic diffusion control, tautomerism. 32 [3] 33

34 protophilic (solvent) 35

See HBA (hydrogen bond acceptor) solvent. 36 rev[3] 37

38 39 40

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protophilicity 1 Special case of nucleophilicity, describing the relative rates of reactions of a series of 2 Lewis bases with a common Brønsted acid. This term has the advantage over the 3 commonly used “kinetic basicity” that philicity and fugality are associated with kinetics, 4 while acidity and basicity are associated with thermodynamics. 5

See [194]. 6 See also Brønsted basicity, nucleophilicity, protofugality. 7

8 prototropic rearrangement (prototropy) 9

See tautomerization. 10 [3] 11

12 pseudo-catalysis 13 Increase of the rate of a reaction by an acid or base present in nearly constant 14 concentration throughout a reaction in solution (owing to buffering or to the use of a 15 large excess), even though that acid or base is consumed during the process, so that 16 the acid or base is not a catalyst and the phenomenon strictly cannot be called catalysis 17 according to the established meaning of these terms in chemical kinetics. 18

Note 1: Although the mechanism of such a process is often closely related to that of 19 a catalysed reaction, it is recommended that the term pseudo-catalysis be used in these 20 and analogous cases. For example, if a Brønsted acid accelerates the hydrolysis of an 21 ester to a carboxylic acid and an alcohol, this is properly called acid catalysis, whereas 22 the acceleration, by the same acid, of the hydrolysis of an amide should be described 23 as pseudo-catalysis because the acid pseudo-catalyst is stoichiometrically consumed 24 during the reaction through formation of an ammonium ion. 25

Note 2: The terms general-acid pseudo-catalysis and general-base pseudo-catalysis 26 may be used as the analogues of general acid catalysis and general base catalysis. 27

Note 3: The terms acid- and base-promoted, acid- and base-accelerated, and acid- 28 and base-induced are sometimes used for reactions that are pseudo-catalysed by or 29 bases. However, the term promotion also has a different meaning in other chemical 30 contexts. 31

rev[3] 32 33 pseudo-first-order rate coefficient 34

See order of reaction, rate coefficient. 35 [3] 36

37 38 39 40

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pseudopericyclic 1 Feature of a concerted transformation in which the primary changes in bonding occur 2 within a cyclic array of atoms but in which one (or more) nonbonding and bonding 3 atomic orbitals interchange roles. 4

Examples: enol-to-enol tautomerism of 4-hydroxypent-3-en-2-one (where the 5 electron pairs in the O–H bond and in the lone pair on the other O are in σ orbitals 6 whereas the other three electron pairs are in p orbitals) and hydroboration (where the 7 B uses an sp2 bonding orbital and a vacant p orbital) 8 9

10 11

Note: Because the atomic orbitals that interchange roles are orthogonal, such a 12 reaction does not proceed through a fully conjugated transition state and is thus not a 13 pericyclic reaction. It is therefore not governed by the rules that express orbital 14 symmetry restrictions applicable to pericyclic reactions. 15

See [354,355]. 16 See also coarctate. 17 rev[3] 18

19 push-pull conjugation 20 Feature of an extended conjugated p system bearing an electron donor group at one 21 end and an electron acceptor group at the other end. 22

See [356]. 23 See also cross-conjugation. 24

25 pyrolysis 26 Thermolysis, usually associated with exposure to a high temperature. 27

See also flash vacuum pyrolysis. 28 [3] 29

30 p-adduct (pi-adduct) 31 Adduct formed by electron-pair donation from a p orbital into a s* orbital, or from a s 32 orbital into a p* orbital, or from a p orbital into a p* orbital. 33

O OH

OOH

C C

B H

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Example: 1

2 Note: Such an adduct has commonly been known as a p complex, but, as the 3

bonding is not necessarily weak, it is better to avoid the term complex, in accordance 4 with the recommendations in this Glossary. 5

See also coordination. 6 [3] 7

8 p-bond (pi bond) 9 Interaction between two atoms whose p orbitals overlap sideways. 10

Note: The designation as p is because the p orbitals are antisymmetric with respect 11 to a defining plane containing the two atoms. 12

See sigma, pi. 13 rev[3] 14

15 p-complex 16

See p-adduct. 17 [3] 18

19 p-electron acceptor 20 Substituent capable of electron withdrawal by resonance (e.g., NO2). 21

See electronic effect, polar effect, p-electron donor, s-constant. 22 rev[3] 23

24 p-electron donor 25 Substituent capable of electron donation by resonance (e.g., OCH3). 26

See electronic effect, polar effect, p-electron acceptor, s-constant. 27 rev[3] 28

29 quantitative structure-activity relationship (QSAR) 30 Regression model to correlate biological activity or chemical reactivity with predictor 31 parameters based on measured or calculated features of molecular structure. 32

See [357,358]. 33 See also correlation analysis. 34

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rev[3] 1 2 quantitative structure-property relationship (QSPR) 3 Regression model to correlate chemical properties such as boiling point or 4 chromatographic retention time with predictor parameters based on measured or 5 calculated features of molecular structure. 6

See [359]. 7 See also correlation analysis. 8

9 quantum yield 10 Number of defined events that occur per photon absorbed by the system. 11

Note 1: The integral quantum yield F is the number of events divided by the number 12 of photons absorbed in a specified wavelength range. 13

Note 2: For a photochemical reaction F(l) is the amount of reactant consumed or 14 product formed divided by the number of photons absorbed at wavelengthl. 15

Note 3: The differential quantum yield for a homogeneous system is 16 17

𝛷(𝜆) = Md𝑥d𝑡M

𝑞*,1[1 − 102(1)] 18

19 where |dx/dt| is the rate of change of a quantity x that measures the progress of a 20

reaction, qp,l is the spectral photon flux (mol or its non-SI equivalent einstein) incident 21 per unit time at wavelength l, and A(l) is the decadic absorbance at the excitation 22 wavelengthl. 23

Note 4: When the quantity x is an amount concentration, it is convenient to use in 24 the denominator the rate (in moles) of photons absorbed per volume. 25

See [9,10]. 26 rev[3] 27

28 radical 29 free radical (obsolete) 30 Molecular entity possessing an unpaired electron. 31

Examples: •CH3, •SnR3, Cl• 32 Note 1: In these formulae the dot, symbolizing the unpaired electron, should be 33

placed so as to indicate the atom of highest spin density, if possible. 34 Note 2: Paramagnetic metal ions are not normally regarded as radicals. However, in 35

the isolobal analogy the similarity between certain paramagnetic metal ions and radicals 36 becomes apparent. 37

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Note 3: Depending upon the core atom that possesses the highest spin density, the 1 radicals can be described as carbon-, oxygen-, nitrogen-, or metal-centred radicals. 2

Note 4: If the unpaired electron occupies an orbital having considerable s or more or 3 less pure p character, the respective radicals are termed s or p radicals. 4

Note 5: The term radical has also been used to designate a substituent group within 5 a molecular entity, as opposed to "free radical", which is now simply called radical. The 6 bound entities may be called groups or substituents, but should no longer be called 7 radicals. 8

See [40,97]. 9 See also diradical. 10 rev[3] 11

12 radical combination 13 Formation of a covalent bond by reaction of one radical with another. 14

See colligation. 15 rev[3] 16

17 radical ion 18 Radical that carries a net electric charge. 19

Note 1: A positively charged radical is called a radical cation (e.g., the benzene 20 radical cation C6H6•+); a negatively charged radical is called a radical anion (e.g., the 21 benzene radical anion C6H6•– or the benzophenone radical anion Ph2C–O•–). 22

Note 2: Unless the positions of unpaired spin and charge can be associated with 23 specific atoms, superscript dot and charge designations should be placed in the order 24 •+ or •–, as suggested by the name radical ion. However, the usage in mass 25 spectrometry is to place the charge symbol before the dot [7]. 26

Note 3: In the first edition of this Glossary it was recommended to place the charge 27 designation directly above the dot. This format is now discouraged because of the 28 difficulty of extending it to ions bearing more than one charge and/or more than one 29 unpaired electron. 30

[3] 31 32 radical pair 33 geminate pair 34 Two radicals in close proximity in solution, within a solvent cage. 35

Note 1: The two radicals may be formed simultaneously by some unimolecular 36 process, e.g., peroxide decomposition or photolysis, or they may have come together 37 by diffusion. 38

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Note 2: While the radicals are together, correlation of the unpaired electron spins of 1 the two species cannot be ignored: this correlation is responsible for the CIDNP 2 phenomenon. 3

See also geminate recombination. 4 See [9,40]. 5 [3] 6

7 radiolysis 8 Cleavage of one or several bonds resulting from exposure to high-energy radiation. 9

Note: The term is also often used loosely to specify the method of irradiation (e.g., 10 pulse radiolysis) used in any radiochemical reaction, not necessarily one involving bond 11 cleavage. 12

[3] 13 14 rate coefficient 15 Empirical constant k in the equation for the rate of a reaction that is expressible by an 16 equation of the form 17 18

v = k[A]a[B]b.... 19 20

Note 1: It is recommended that the term rate constant be confined to reactions that 21 are believed to be elementary reactions. 22

Note 2: When a rate coefficient relates to a reaction occurring by a composite 23 mechanism, it may vary not only with temperature and pressure but also with the 24 concentration of reactants. For example, in the case of a unimolecular gas reaction the 25 rate at sufficiently high pressures is given by 26 27

v = k[A] 28 29 whereas at low pressures the rate expression is 30 31

v = k'[A]2 32 33 Similarly, for a second-order reaction, the rate is given by 34 35

v = k2[A][B] 36 37 But under conditions where [B] remains constant at [B]0, as when B is a catalyst or is 38 present in large excess, 39 40

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v = k2[A][B]0 = k[A] 1 2 where the rate coefficient k = k2[B]0, which varies with [B]0. 3

Such a rate coefficient k, which varies with the concentration [B], is called a first-4 order rate coefficient for the reaction, or a pseudo first-order rate constant even though 5 it is not a rate constant. 6

Note: Rate constant and rate coefficient are often used as synonyms, see [12], 7 section 2.12, p.63. 8

See [13]. 9 See order of reaction. 10 rev[3] 11

12 rate constant 13 k 14 Term generally used for the rate coefficient of a reaction that is believed to be an 15 elementary reaction. See [13]. 16

Note 1: In contrast to a rate coefficient, a rate constant should be independent of 17 concentrations, but in general both rate constant and rate coefficient vary with 18 temperature. 19

Note 2: Rate constant and rate coefficient are often used as synonymous, see [12], 20 section 2.12, p.63. 21

See order of reaction. 22 rev[3] 23

24 rate-controlling step 25

See rate-determining states, rate-limiting step. 26 rev[3] 27

28 rate law 29 empirical differential rate equation 30 Expression for the rate of reaction in terms of concentrations of chemical species and 31 constant parameters (normally rate coefficients and partial orders of reaction) only. 32

For examples of rate laws see the equations under kinetic equivalence, and under 33 steady state. 34

[3] 35 36 rate-limiting step 37 rate-controlling step 38 rate-determining step 39

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Step in a multistep reaction that is the last step in the sequence whose rate constant 1 appears in the rate equation. 2

See [13]. 3 Example: Two-step reaction of A with B to give intermediate C, which then reacts 4

further with D to give products: 5

6 If [C] reaches a steady state, then the observed rate is given by 7 8

v = –d[A]/dt = k1k2[A][B][D]/(k–1 + k2[D]) 9 10

Case 1: If k2[D] >> k–1,then the observed rate simplifies to 11 12

v = –d[A]/dt = k1[A][B] 13 14 Because k2 disappears from the rate equation and k1 is the last rate constant to remain, 15 step (1) is said to be rate-limiting. 16

Case 2: If k2[D] << k–1, then the observed rate is given by 17 18

v = k1k2[A][D]/k–1 = Kk2[A][B][D] 19 20 where K, equal to k1/k–1, is the equilibrium constant for the pre-equilibrium (Step 1). 21 Because k2 remains in the rate equation, Step 2 is said to be rate-limiting. Notice that 22 in this case, where Step 2 involves another reactant D, which step is rate-limiting can 23 depend on [D]: Step 1 at high [D] and Step 2 at low [D]. 24 25

Specific example: Acid-catalyzed bromination of a methyl ketone 26 27

28 29 where k1 = k1'[H+], k–2 = k–2'[H+], k3 = k3'[Br2], k–3 = k–3'[Br–]. 30 31 According to the steady-state approximation, 32 33

v = kobs[RCOCH3] = k1k2k3k4[RCOCH3]/{k2k3k4 + k–1k3k4 + k–1k–2(k–3 + k4)}. 34 35

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or kobs = k1k2k3k4/{k2k3k4 + k–1k3k4 + k–1k–2(k–3 + k4)}. 1 2 If k4 >> k–3, k–3 can be ignored in the parentheses, so that kobs simplifies to 3 k1k2k3k4/{k2k3k4 + k–1k3k4 + k–1k–2k4}. Then if k3 >> k–2, kobs simplifies further to 4 k1k2k3k4/{k2k3k4 + k–1k3k4}, where k3k4 cancels and kobs becomes k1k2/{k–1 + k2}, so that 5 k2 is the last rate constant remaining in kobs. The second step is rate-limiting, and the 6 rate is independent of k3 or of [Br2]. 7

Note 1: Although the expressions rate-controlling, rate-determining, and rate-limiting 8 are often regarded as synonymous, rate-limiting is to be preferred, because in Case 2 9 all three rate constants enter into the rate equation, so that all three are rate-controlling 10 and rate-determining, but the first step is not rate-limiting. 11

Note 2: If the concentration of any intermediate builds up to an appreciable extent, 12 then the steady-state approximation no longer holds, and the reaction should be 13 analyzed as though that intermediate is the reactant. 14

Note 3: It should be noted that a catalytic cycle does not have a rate-determining 15 step. Instead, under steady-state conditions, all steps proceed at the same rate 16 because the concentrations of all intermediates adjust so as to offset the differences in 17 the corresponding rate constants [360]. 18

See also Gibbs energy diagram, microscopic diffusion control, mixing control. 19 rev[3] 20

21 rate of reaction 22 v 23 (unit: mol dm–3 s–1 or mol L–1 s–1) 24 For the general chemical reaction 25 26

a A + b B =!!!p P + q Q ... 27 28 occurring under constant-volume conditions, without an appreciable build-up of reaction 29 intermediates, the rate of reaction v is defined as 30 31

𝑣 = −1𝑎d[A]d𝑡 = −

1𝑏d[B]d𝑡 = −

1𝑝d[P]d𝑡 = −

1𝑞 d[Q]d𝑡 32

33 where symbols inside square brackets denote concentrations (conventionally 34 expressed in unit mol dm–3). The symbols R and r are also used instead of v. It is 35 recommended that the unit of time be the second. 36

Example: 37 38

Cr2O72– + 3 (CH3)2CHOH + 8 H+ = 2 Cr3+ + 3 (CH3)2C=O + 7 H2O 39

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1

𝑣 = −d[Cr#O5#7]

d𝑡 = −13d[(CH8)#CHOH]

d𝑡 = 12d[𝐶𝑟89]d𝑡 =

13d[(CH8)#C = O]

d𝑡 2

3 Note: For a stepwise reaction this definition of rate of reaction will apply only if there 4

is no accumulation of intermediate or formation of side products. It is therefore 5 recommended that the term rate of reaction be used only in cases where it is 6 experimentally established that these conditions apply. More generally, it is 7 recommended that, instead, the terms rate of disappearance or rate of consumption of 8 A (i.e., –d[A]/dt) or rate of appearance of P (i.e., d[P]/dt) be used, depending on the 9 particular chemical species that is actually observed. In some cases reference to the 10 chemical flux observed may be more appropriate. 11

See [13]. 12 See also chemical relaxation, lifetime, order of reaction. 13 rev[3] 14

15 reaction coordinate 16 Parameter that changes during the conversion of one (or more) reactant molecular 17 entities into one (or more) product molecular entities and whose value can be taken as 18 a measure of the progress along a minimum-energy reaction path. 19

Note 1: The term "reaction coordinate" is often used to refer to a geometric variable 20 itself (typically a bond distance or bond angle, or a combination of distances and/or 21 angles) as well as (or instead of) the value of that variable. Although strictly incorrect, 22 this usage is very commonly encountered. 23

Note 2: In cases where the location of the transition structure is unknown, an internal 24 coordinate of the system (e.g., a geometric variable or a bond order, or an energy gap 25 between reactant-like and product-like valence-bond structures) is often selected as a 26 reaction coordinate. A potential-energy profile obtained by energy minimization over 27 other coordinates for a succession of fixed values of an arbitrary reaction coordinate is 28 not guaranteed to pass through the transition structure unless it is a continuous function 29 of that reaction coordinate. Similarly, a free-energy profile obtained as a potential of 30 mean force with respect to an arbitrary reaction coordinate is not guaranteed to pass 31 through the lowest-energy transition state. 32

Note 3: “Reaction coordinate” is sometimes used as an undefined label for the 33 horizontal axis of a potential-energy profile or a Gibbs energy diagram. 34

See [349,361]. 35 See also Gibbs energy diagram, potential-energy profile, potential-energy surface. 36 rev[3] 37

38 reaction path 39 (1) Synonym for mechanism. 40

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(2) Trajectory on the potential-energy surface. 1 rev[3] 2

3 reaction step 4 Elementary reaction constituting one of the stages of a stepwise reaction in which a 5 reaction intermediate (or, for the first step, the reactants) is converted into the next 6 reaction intermediate (or, for the last step, the products) in the sequence of 7 intermediates between reactants and products. 8

[3] 9 10 reactive intermediate 11 intermediate 12 13 reactivity (n.), reactive (adj.) 14 Kinetic property of a chemical species by which (for whatever reason) it has a different 15 rate constant for a specified elementary reaction than some other (reference) species. 16

Note 1: The term has meaning only by reference to some explicitly stated or implicitly 17 assumed set of conditions. It is not to be used for reactions or reaction patterns of 18 compounds in general. 19

Note 2: Term also used more loosely as a phenomenological description not 20 restricted to elementary reactions. When applied in this sense, the property under 21 consideration may reflect not only rate constants but also equilibrium constants. 22

See also stable, unreactive, unstable. 23 [3] 24

25 reactivity index 26 Numerical quantity derived from quantum-mechanical model calculations or from a 27 linear Gibbs-energy relationship (linear free-energy relationship) that permits the 28 prediction or correlation of relative reactivities of different molecular sites. 29

Note: Many indices are in use, based on a variety of theories and relating to various 30 types of reaction. The more successful applications have been to the substitution 31 reactions of conjugated systems, where relative reactivities are determined largely by 32 changes of p-electron energy and p-electron density. 33

rev[3] 34 35 reactivity-selectivity principle (RSP) 36 Idea that the more reactive a reagent is, the less selective it is. 37

Note: There are many examples in which the RSP is followed, but there are also 38 many counterexamples. Although the RSP is in accord with intuitive feeling, it is now 39

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clear that selectivity can decrease, increase, or remain constant as reactivity increases, 1 so that the RSP is unreliable as a guide to reactivity. 2

See [126,346,362,363,364,365]. 3 rev[3] 4

5 rearrangement 6

See degenerate rearrangement, molecular rearrangement, sigmatropic 7 rearrangement. 8

[3] 9 10 reduction 11 (1) Transfer of one or more electrons to a molecular entity, usually inorganic. 12 (2) Decrease in the oxidation number of any atom within any substrate [344]. 13 (3) Loss of oxygen or halogen and/or gain of hydrogen of an organic substrate. 14

See oxidation. 15 rev[3] 16

17 reductive elimination 18 Reverse of oxidative addition. 19

[3] 20 21 regioselectivity (n.), regioselective (adj.) 22 Property of a reaction in which one position of bond making or breaking occurs 23 preferentially over all other possible positions. 24

Note 1: The resulting regioisomers are constitutional isomers. 25 Note 2: Reactions are termed completely (100 %) regioselective if the discrimination 26

is complete, or partially (x %) if the product of reaction at one site predominates over 27 the product of reaction at other sites. The discrimination may also be referred to semi-28 quantitatively as high or low regioselectivity. 29

Note 3: Historically the term was restricted to addition reactions of unsymmetrical 30 reagents to unsymmetrical alkenes. 31

Note 4: In the past, the term regiospecificity was proposed for 100 % regioselectivity. 32 This terminology is not recommended, owing to inconsistency with the terms 33 stereoselectivity and stereospecificity. 34

See [366,367]. 35 See also chemoselectivity. 36 rev[3] 37

38 Reichardt ET parameter 39

See Dimroth-Reichardt ET(30) parameter. 40

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1 relaxation 2 Passage of a system that has been perturbed from equilibrium, by radiation excitation 3 or otherwise, toward or into thermal equilibrium with its environment. 4

See [9]. 5 See also chemical relaxation. 6 rev[3] 7

8 reorganization energy 9 Gibbs energy required to distort the reactants (and their associated solvent molecules) 10 from their relaxed nuclear configurations to the relaxed nuclear configurations of the 11 products (and their associated solvent molecules). 12

Note 1: This approach was originally formulated for one-electron transfer reactions, 13 A + D → A–· + D+·, in the framework of the Marcus equation, assuming weak coupling 14 between the reactants [368]. 15

Note 2: Reorganization energy is not the same as distortion energy, which is the 16 energy required to distort the reactants to the nuclear configuration of the transition 17 state. 18

Note 3: This approach has been extended to enzyme-catalysed reactions [369]. 19 Note 4: Marcus theory has been shown to be valid for some complex reactions 20

(cycloaddition, SN2), even though the weak-coupling assumption is clearly not valid. In 21 these cases the reorganization energy (in terms of activation strain) is counteracted by 22 stabilizing interactions (electrostatic and orbital) [166]. 23

See also distortion interaction model, intrinsic barrier, Marcus equation. 24 rev[3] 25

26 resonance 27 Representation of the electronic structure of a molecular entity in terms of contributing 28 Lewis structures. 29

Note 1: Resonance means that the wavefunction is represented by mixing the 30 wavefunctions of the contributing Lewis structures. 31

Note 2: The contributing Lewis structures are represented as connected by a double-32 headed arrow (↔), rather than by the double arrow (⇄) representing equilibrium 33 between species. 34

Note 3: This concept is the basis of the quantum-mechanical valence-bond methods. 35 The resulting stabilization is linked to the quantum-mechanical concept of resonance 36 energy. The term resonance is also used to refer to the delocalization phenomenon 37 itself. 38

Note 4: This term has a completely different meaning in physics. 39 See [75,370]. 40

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See also [371]. 1 rev[3] 2

3 resonance effect 4 Experimentally observable influence (on reactivity, etc.) of a substituent through 5 electron delocalization to or from the substituent. 6

See [126,173,191]. 7 See also inductive effect. 8 rev[3] 9

10 resonance energy 11 Difference in potential energy between the actual molecular entity and the contributing 12 Lewis structure of lowest potential energy. 13

Note: The resonance energy cannot be measured experimentally, but only 14 estimated, since contributing Lewis structures are not observable molecular entities. 15

See resonance. 16 rev[3] 17

18 resonance form 19 resonance structure, contributing structure, canonical form 20 One of at least two Lewis structures,with fixed single, double, and triple bonds, that is 21 a contributing structure to the valence-bond wavefunction of a molecule that cannot be 22 described by a single Lewis structure. 23

Note 1: Although the valence-bond wavefunction is a linear combination of the 24 wavefunctions of the individual resonance forms, the coefficients and the relative 25 contributions of the various resonance forms are usually kept qualitative. For example, 26 the major resonance forms for the conjugate base of acetone are CH2=C(CH3)–O– and 27 H2C––C(CH3)=O, with the former contributing more. 28

Note 2: Resonance forms are connected by a double-headed arrow (↔). This must 29 not be confused with the double arrow connecting species in equilibrium (⇌). 30

See also delocalization, Kekulé structure, resonance. 31 32 resonance hybrid 33 Molecular entity whose electronic structure is represented as the superposition of two 34 or more resonance forms (or Lewis structures) with different formal arrangements of 35 electrons but identical arrangements of nuclei. 36

Note 1: Whereas each contributing resonance form represents a localized 37 arrangement of electrons which, considered by itself, would imply different bond lengths 38 (say) for formal single and double bonds, resonance between two or more contributors 39 requires each to have the same geometry, namely that of the resulting hybrid, which 40

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represents a delocalized arrangement of electrons. A particular bond in the hybrid may 1 have a length that is, loosely, an “average” of formal single-bond and double-bond 2 values implied by its contributing individual resonance forms, but the hybrid does not 3 oscillate among these as if they were in equilibrium. 4

Note 2: The resonance forms are connected by a double-headed arrow (↔), rather 5 than by the double arrow (⇄) representing equilibrium between species. 6 7 retrocycloaddition 8 deprecated 9 Cycloelimination. 10 [3] 11 12 Ritchie equation 13 Linear Gibbs-energy relation (linear free-energy relation) 14 15

lg {kN} = lg {k0} + N+ 16 17 applied to the reactions between nucleophiles and certain large and relatively stable 18 organic cations, e.g., arenediazonium, triarylmethyl, and aryltropylium cations, in 19 various solvents, where {kN} is the (reduced) second-order rate constant for reaction of 20 a given cation with a given nucleophilic system (i.e., given nucleophile in a given 21 solvent). {k0} is the (reduced) first-order rate constant for the same cation with water in 22 water, and N+ is a parameter characteristic of the nucleophilic system and independent 23 of the electrophilic reaction partner. 24

The discrepancy between second-order and first-order rate constants must be 25 reconciled by writing the equation with arguments of the logarithms of dimension 1, i.e., 26 by using reduced rate constants (as denoted by the curly brackets in the defining 27 equation): 28 29

lg [kN/(dm3 mol-1s-1)] = lg (k0/s-1) + N+ 30 31

Note 1: A surprising feature of the equation is the absence of a coefficient of N+ 32 characteristic of the substrate (cf. the s in the Swain-Scott equation), even though 33 values of N+ vary over 13 decadic log (lg) units. The equation thus involves a gigantic 34 breakdown of the reactivity-selectivity principle. 35

Note 2: The Ritchie equation is a special case of the more general Mayr-Patz 36 equation. 37

See [193,372,373]. See also [125,374]. 38 rev[3] 39

40

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r-value (rho-value) 1 Quantitative measure of the susceptibility of the rate constant or equilibrium constant 2 of an organic reaction to the influence of substituent groups, usually on an aromatic 3 ring. 4

Note 1: Defined by Hammett to describe the effects of substituents at the meta- and 5 para-positions on rate or equilibrium of a reaction on the side chain of a substituted 6 benzene, the empirical equation has the general form 7 8

lg(kX/kH) or lg(KX/KH) = rsX 9 10 in which sX is a constant characteristic of the substituent X and of its position in the 11 reactant molecule. 12

Note 2: More generally (not only for aromatic series), r-values (modified with 13 appropriate subscripts and superscripts) are used to designate the susceptibility to 14 substituent effects of reactions of families of organic compounds, as given by the 15 modified set of s-constants in an empirical correlation. 16

Note 3: Reactions with a positive r-value are accelerated (or the equilibrium 17 constants are increased) by substituents with positive s-constants. Since the sign of s 18 was defined so that substituents with a positive s increase the acidity of benzoic acid, 19 such substituents are generally described as attracting electrons away from the 20 aromatic ring. It follows that reactions with a positive r-value involve a transition state 21 (or reaction product) with an increased electron density at the reactive site of the 22 substrate. 23

See also Hammett equation, s-constant, Taft equation. 24 rev[3] 25

26 rs-equation (rho-sigma equation) 27

See Hammett equation, r-value, s-constant, Taft equation. 28 [3] 29

30 salt effect 31

See kinetic electrolyte effect. 32 [3] 33

34 saturation transfer 35

See magnetization transfer. 36 rev[3] 37

38 Saytzeff rule 39 Preferential removal of a hydrogen from the b carbon that has the fewest hydrogens in 40

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dehydrohalogenation of secondary and tertiary haloalkanes. 1 Note 1: The rule was originally formulated by A. Saytzeff (Zaitsev) to generalize the 2

orientation in b-elimination reactions of haloalkanes. It has been extended and 3 modified, as follows: When two or more olefins can be produced in an elimination 4 reaction, the thermodynamically most stable alkene will predominate. 5

Note 2: Exceptions to the Saytzeff rule are exemplified by the Hofmann rule. 6 See [375]. 7 See also Markovnikov rule. 8 [3] 9

10 scavenger 11 Substance that reacts with (or otherwise removes) a trace component (as in the 12 scavenging of trace metal ions) or traps a reactive intermediate. 13

See also inhibition. 14 [3] 15

16 selectivity 17 Discrimination shown by a reagent in competitive attack on two or more substrates or 18 on two or more positions or diastereotopic or enantiotopic faces of the same substrate. 19

Note 1: Selectivity is quantitatively expressed by the ratio of rate constants of the 20 competing reactions, or by the decadic logarithm of such a ratio. 21

Note 2: In the context of aromatic substitution (usually electrophilic, for 22 monosubstituted benzene derivatives), the selectivity factor Sf (expressing 23 discrimination between p- and m-positions in PhZ) is defined as 24 25

Sf = lg (pfZ/mfZ) 26 27 where the partial rate factors pfZ and mfZ express the reactivity of para and meta 28 positions in the aromatic compound PhZ relative to that of a single position in benzene. 29

See [347]. 30 See also isoselective relationship, partial rate factor, regioselectivity, 31

stereoselectivity. 32 rev[3] 33

34 self-assembly 35 Process whereby a system of single-molecule components spontaneously forms an 36 organized structure, owing to molecular recognition. 37 38 shielding 39 Extent to which the effective magnetic field is reduced for a nucleus in a molecule 40

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immersed in an external magnetic field, relative to that experienced by a bare nucleus 1 in that field. 2

Note 1: The reduction is due to the circulation of the electrons around the observed 3 and the neighbouring nuclei. The external field induces a magnetic moment that is 4 oriented in the opposite direction to the external field, so that the local field at the central 5 nucleus is weakened, although it may be strengthened at other nuclei (deshielding). 6

Note 2: This phenomenon is the origin of the structural dependence of the resonance 7 frequencies of the nuclei. 8

See also chemical shift. 9 [3] 10

11 shift reagent 12 Paramagnetic substance that induces an additional change of the NMR resonance 13 frequency of a nucleus near any site in a molecule to which the substance binds. 14

See chemical shift. 15 16 sigma, pi 17

See s, p 18 rev[3] 19

20 sigmatropic rearrangement 21 Molecular rearrangement that involves both the creation of a new s bond between 22 atoms previously not directly linked and the breaking of an existing s bond. 23

Note 1: There is normally a concurrent relocation of p bonds in the molecule 24 concerned, but neither the number of p bonds nor the number of s bonds changes. 25

Note 2: The transition state of such a reaction may be visualized as an association 26 of two fragments connected at their termini by two partial s bonds, one being broken 27 and the other being formed as, for example, the two allyl fragments in (a'). Considering 28 only atoms within the (real or hypothetical) cyclic array undergoing reorganization, if the 29 numbers of these in the two fragments are designated i and j, then the rearrangement 30

is said to be a sigmatropic change of order [i,j] (conventionally i!2!j). Thus rearrangement 31

(a) is of order [3,3], whereas rearrangement (b) is a [1,5]sigmatropic shift of hydrogen. 32 (By convention the square brackets [...] here refer to numbers of atoms, in contrast with 33 current usage in the context of cycloaddition.) 34

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1 2 The descriptors a and s (antarafacial and suprafacial) may also be annexed to the 3 numbers i and j; (b) is then described as a [1s,5s] sigmatropic rearrangement, since it is 4 suprafacial with respect to both the hydrogen atom and the pentadienyl system: 5 6

7 8

See also cycloaddition, tautomerization. 9 rev[3] 10

11 silylene 12 (1) Generic name for H2Si: and substitution derivatives thereof, containing an 13 electrically neutral bivalent silicon atom with two non-bonding electrons. (The definition 14 is analogous to that given for carbene.) 15 (2) The silanediyl group (H2Si<), analogous to the methylene group (H2C<). 16

[3] 17 18 single-electron transfer mechanism (SET) 19 Reaction mechanism characterized by the transfer of a single electron between two 20 species, occurring in one of the steps of a multistep reaction. 21

[3] 22 23 single-step reaction 24 one-step reaction 25 Reaction that proceeds through a single transition state (or no transition state). 26

[3] 27 28 29

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skeletal formula 1 bond-line formula 2 Two-dimensional representation of a molecular entity in which bonds are indicated as 3 lines between vertices representing octet carbon atoms with attached hydrogens 4 omitted and in which other atoms are represented by their chemical symbols. 5

Examples: ethanol, cyclohexene, 4-pyridone 6 7

8 9

See line formula. 10 11 Slater-type orbital (STO) 12

Function centred on an atom for which the radial dependence has the form f(r) 3!rn–1 13

exp(–zr), used to approximate atomic orbitals in the LCAO-MO method. 14 Note 1: n is the effective principal quantum number and z is the orbital exponent 15

(screening constant) derived from empirical considerations. 16 Note 2: The angular dependence is usually introduced by multiplying the radial 17

function by a spherical harmonic Y1m(q,f). 18 Note 3: Owing to difficulties in computing the integrals of STOs analytically for 19

molecules with more than two atoms they are often replaced by linear combinations of 20 Gaussian orbitals. 21

See [8]. 22 rev[3] 23

24 solvation 25 Stabilizing interaction between a solute (or solute moiety) and the solvent. 26

Note: Such interactions generally involve electrostatic forces and van der Waals 27 forces, as well as chemically more specific effects such as hydrogen bond formation. 28

See also cybotactic region. 29 [3] 30

31 solvatochromic relationship 32 Linear Gibbs-energy relationship (linear free-energy relationship) based on 33 solvatochromism. 34

OH

N

O

H

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See also Dimroth-Reichardt ET parameter, Kamlet-Taft solvent parameters. 1 [3] 2

3 solvatochromism 4 Pronounced change in position and sometimes intensity of an electronic absorption or 5 emission band, accompanying a change in the polarity of the medium. 6

Note: Negative (positive) solvatochromism corresponds to a hypsochromic shift 7 (bathochromic shift) with increasing solvent polarity. 8

See [9,17,376]. 9 See also Dimroth-Reichardt ET parameter, Z-value. 10 [3] 11

12 solvatomers 13 Isomers that differ in their solvation environment. 14

Note 1: Because the solvation environment fluctuates rapidly, solvatomers 15 interconvert rapidly. 16

Note 2: Species that differ in the type of solvent molecules should not be called 17 solvatomers, because they are not isomers. 18 19 solvent parameter 20 Quantity that expresses the capability of a solvent for interaction with solutes, based on 21 experimentally determined physicochemical quantities, in particular: relative 22 permittivity, refractive index, rate constants, Gibbs energies and enthalpies of reaction, 23 and ultraviolet-visible, infrared, and NMR spectra. 24

Note 1: Solvent parameters are used in correlation analysis of solvent effects, either 25 in single-parameter or in multiple-parameter equations. 26

Note 2: Solvent parameters include those representing a bulk property, such as 27 relative permittivity (dielectric constant) as well as those that describe a more localized 28 solute/solvent interaction, such as hydrogen-bonding acceptance or donation and 29 Lewis acid/base adduct formation. 30

See [17,377]. 31 See also: acceptor number, Catalán solvent parameters, Dimroth-Reichardt ET 32

parameter, Grunwald-Winstein equation, Kamlet-Taft solvent parameters, Laurence 33 solvent parameters, Koppel-Palm solvent parameters, linear solvation energy 34 relationship, Z-value. 35

rev[3] 36 37 solvolysis 38 Reaction with solvent. 39

Note 1: Such a reaction generally involves the rupture of one or more bonds in the 40

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solute. More specifically the term is used for substitution, elimination, and fragmentation 1 reactions in which a solvent species serves as nucleophile or base. 2

Note 2: A solvolysis can also be classified as a hydrolysis, alcoholysis, or 3 ammonolysis, etc., if the solvent is water, alcohol, or ammonia, etc. 4

Note 3: Often a solvolysis is a nucleophilic substitution (usually SN1, accompanied 5 by E1 elimination), where the nucleophile is a solvent molecule. 6

rev[3] 7 8 SOMO 9 Singly Occupied Molecular Orbital (such as the half-filled HOMO of a radical). 10

See also frontier orbitals. 11 [3] 12

13 special salt effect 14 Steep increase of the rate of certain solvolysis reactions observed at low concentrations 15 of some non-common-ion salts. 16

Note: The effect is attributed to trapping of an intimate ion pair that would revert to 17 reactant in the absence of the salt. 18

See also kinetic electrolyte effect. 19 rev[3] 20

21 specific catalysis 22 Acceleration of a reaction by a unique catalyst, rather than by a family of related 23 substances. 24

Note: The term is most commonly used in connection with specific hydrogen-ion or 25 hydroxide-ion (lyonium ion or lyate ion) catalysis. 26

See also general acid catalysis, general base catalysis, pseudo-catalysis. 27 [3] 28

29 spectator mechanism 30 Pre-association mechanism in which one of the molecular entities, C, is already present 31 in an encounter pair with A during formation of B from A, but does not assist the 32 formation of B, e.g., 33 34

35 36

Note: The formation of B from A may itself be a bimolecular reaction with some other 37 reagent. Since C does not assist the formation of B, it is described as being present as 38

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a spectator. 1 See also microscopic diffusion control. 2 [3] 3

4 spin adduct 5

See spin trapping. 6 [3] 7

8 spin counting 9

See spin trapping. 10 [3] 11

12 spin density 13 Unpaired electron density at a position of interest, usually at carbon, in a radical or a 14 triplet state. 15

Note: Spin density is often measured experimentally by electron paramagnetic 16 resonance/electron spin resonance (EPR/ ESR) spectroscopy through hyperfine 17 splitting of the signal by neighbouring magnetic nuclei. 18

See also radical centre. 19 [3] 20

21 spin label 22 Stable paramagnetic group (typically an aminoxy radical, R2NO•) that is attached to a 23 part of a molecular entity whose chemical environment may be revealed by its electron 24 spin resonance (ESR) spectrum. 25

Note: When a paramagnetic molecular entity is used without covalent attachment to 26 the molecular entity of interest, it is frequently referred to as a spin probe. 27

[3] 28 29 spin trapping 30 Formation of a more persistent radical from interaction of a transient radical with a 31 diamagnetic reagent. 32

Note 1: The product radical accumulates to a concentration where detection and, 33 frequently, identification are possible by EPR/ESR spectroscopy. 34

Note 2: The key reaction is usually one of attachment; the diamagnetic reagent is 35 said to be a spin trap, and the persistent product radical is then the spin adduct. The 36 procedure is referred to as spin trapping, and is used for monitoring reactions involving 37 the intermediacy of reactive radicals at concentrations too low for direct observation. 38 Typical spin traps are C-nitroso compounds and nitrones, to which reactive radicals will 39 rapidly add to form aminoxy radicals. 40

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Example: 1

2 Note 3: A quantitative development in which essentially all reactive radicals 3

generated in a particular system are intercepted has been referred to as spin counting. 4 Note 4: Spin trapping has also been adapted to the interception of radicals generated 5

in both gaseous and solid phases. In these cases the spin adduct is in practice 6 transferred to a liquid solution for EPR/ESR observation. 7

[3] 8 9 stable 10 Having a lower standard Gibbs energy, compared to a reference chemical species. 11

Note 1: Quantitatively, in terms of Gibbs energy, a chemical species A is more stable 12 than its isomer B if ∆rG° is positive for the (real or hypothetical) reaction A → B. 13

Note 2: For the two reactions 14 15

P → X + Y ∆rG1° 16 and Q → X + Z ∆rG2° 17 18 if ∆rG1° > ∆rG2°, then P is more stable relative to its product Y than is Q relative to Z. 19

Note 3: Both in qualitative and quantitative usage the term stable is therefore always 20 used in reference to some explicitly stated or implicitly assumed standard. 21

Note 4: The term should not be used as a synonym for unreactive or less reactive 22 since this confuses thermodynamics and kinetics. A relatively more stable chemical 23 species may be more reactive than some reference species towards a given reaction 24 partner. 25

See also inert, unstable. 26 [3] 27

28 stationary state 29

(1) (in quantum mechanics): Wavefunction whose probability density |Y |2 remains 30 constant and whose observable properties do not evolve with time. 31 (2) (in kinetics): See steady state. 32

[3] 33 34 steady state (or stationary state) 35 (1) Approximation that the kinetic analysis of a complex reaction involving unstable 36

N+

O–

+ RN

O

R

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intermediates in low concentration can be simplified by setting the rate of change of 1 each such intermediate equal to zero, so that the rate equation can be expressed as a 2 function of the concentrations of chemical species present in macroscopic amounts. 3

For example, if X is an unstable intermediate in the reaction sequence: 4

5 6 Since [X] is negligibly small, d[X]/dt, the rate of change of [X], can be set equal to zero. 7 The steady state approximation then permits solving the following equation 8 9

d[X]/dt = k1[A] – k–1[X] – k2[X][C] = 0 10 11 to obtain the steady-state [X]: 12 13

[X] = k1[A]/(k–1 + k2[C]) 14 15 whereupon the rate of reaction is expressed: 16 17

d[D]/dt = k2[X][C] = k1k2[A][C]/(k–1 + k2[C]) 18 19

Note: The steady-state approximation does not imply that [X] is even approximately 20 constant, only that its absolute rate of change is very much smaller than that of [A] and 21 [D]. 22 (2) Regime in a stirred flow reactor such that all concentrations are independent of time. 23

See [13]. 24 [3] 25

26 stepwise reaction 27 Chemical reaction with at least one reaction intermediate and involving at least two 28 consecutive elementary reactions. 29

See also composite reaction, reaction step. 30 [3] 31

32 stereoelectronic 33 Pertaining to the dependence of the properties (especially energy or reactivity) of a 34 molecular entity or of a transition state on the relative disposition of electron pairs owing 35 to the nuclear geometry. 36

Note: Stereoelectronic effects are ascribed to the differing overlaps of atomic orbitals 37 in different conformations. 38

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See [11]. 1 [3] 2

3 stereogenic centre 4 Atom within a molecule bearing groups such that interchanging any two of them leads 5 to a stereoisomer of the original molecule. 6

See [11,378]. 7 8 stereoisomers 9 Isomers that have the same bonds (connectivity) but differ in the arrangement of their 10 atoms and cannot be interconverted by rapid rotation around single bonds. 11

See [11]. 12 13 stereoselectivity (stereoselective) 14 Preferential formation in a chemical reaction of one stereoisomer over another. 15

Note 1: When the stereoisomers are enantiomers, the phenomenon is called 16 enantioselectivity and is quantitatively expressed by the enantiomeric excess or 17 enantiomeric ratio; when they are diastereoisomers, it is called diastereoselectivity and 18 is quantitatively expressed by the diastereomeric excess or diastereomeric ratio. 19

Note 2: Reactions are termed 100 % stereoselective if the preference is complete, 20 or partially (x %) stereoselective if one product predominates. The preference may also 21 be referred to semiquantitatively as high or low stereoselectivity 22

See [11,140]. 23 [3] 24

25 stereospecificity (stereospecific) 26 Property of those chemical reactions in which different stereoisomeric reactants are 27 converted into different stereoisomeric products. 28

Example: electrocyclization of trans,cis,trans-octa-2,4,6-triene produces cis-5,6-29 dimethylcyclohexa-1,3-diene, whereas cis,cis,trans-octa-2,4,6-triene produces racemic 30 trans-5,6-dimethylcyclohexa-1,3-diene. 31 32

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1 2

Note 1: A stereospecific process is necessarily stereoselective but not all 3 stereoselective processes are stereospecific. Stereospecificity may be total (100 %) or 4 partial. 5

Note 2: The term is also applied to situations where reaction can be performed with 6 only one stereoisomer. For example the exclusive formation of racemic trans-1,2-7 dibromocyclohexane upon bromination of cyclohexene is a stereospecific process, 8 even though the analogous reaction with (E)-cyclohexene has not been performed. 9

Note 3: Stereospecificity does NOT mean very high stereoselectivity. This usage is 10 unnecessary and is strongly discouraged. 11

See [11,140]. 12 For the term stereospecific polymerization see [379]. 13 rev[3] 14

15 steric-approach control 16 Situation in which the stereoselectivity of a reaction under kinetic control is governed 17 by steric hindrance to attack of the reagent, which is directed to the less hindered face 18 of the molecule. 19

Note: Partial bond making at the transition state must be strong enough for steric 20 control to take place, but the transition state should not be so close to products that the 21 steric demand of the reagent at the transition state is the same as the steric demand of 22 the group as present in the product. 23

An example is LiAlH4 reduction of 3,3,5-trimethylcyclohexan-1-one, where steric 24 hindrance by an axial methyl directs hydride addition to the equatorial position, even 25 though the more stable product has the H axial and the OH equatorial. 26

See also product-development control. 27 rev[3] 28

29 30 31

CH3

CH3CH3

CH3

CH3

CH3CH3

+

CH3CH3

CH3

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steric effect 1 Consequences for molecular geometry, thermochemical properties, spectral features, 2 solvation, or reaction rates resulting from the fact that atoms repel each other at close 3 distance. The repulsion is due to the quantum-mechanical Pauli exclusion principle. 4 Substitution of hydrogen atoms by groups with a larger van der Waals radius may lead 5 to situations where atoms or groups of atoms repel each other, thereby affecting 6 distances and angles. 7

Note 1: It is in principle difficult to separate the steric effect from other electronic 8 effects. 9

Note 2: For the purpose of correlation analysis or linear Gibbs-energy relations 10 (linear free-energy relations) various scales of steric parameters have been proposed, 11 notably A values, Taft's ES and Charton's v scales. 12

Note 3: A steric effect on a rate process may result in a rate increase (steric 13 acceleration) or a decrease (steric retardation) depending on whether the transition 14 state or the reactant state is more affected by the steric effect. 15

Note 4: Bulky groups may also attract each other if at a suitable distance. 16 See [290,380]. 17 See Taft equation, van der Waals forces. 18 rev[3] 19

20 steric hindrance 21 Steric effect whereby the crowding of substituents around a reaction centre retards the 22 attack of a reagent. 23

rev[3] 24 25 stopped flow 26 Technique for following the kinetics of reactions in solution (usually in the millisecond 27 time range) in which two, or more, reactant solutions are rapidly mixed by being forced 28 through a mixing chamber. The flow of the mixed solution along a uniform tube is then 29 suddenly arrested. At a fixed position along the tube the solution is monitored as a 30 function of time following the stoppage of the flow by a method with a rapid response 31 (e.g., optical absorption spectroscopy). 32

See mixing control. 33 rev[3] 34

35 strain 36 Feature of a molecular entity or transition structure for which the energy is increased 37 because of unfavourable non-bonded (steric) interactions, bond lengths, bond angles, 38 or dihedral angles (torsional strain), relative to a standard. 39

Note 1: The strain energy is quantitatively defined as the standard enthalpy of a 40

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structure relative to that of a strainless structure (real or hypothetical) made up from the 1 same atoms with the same types of bonding. 2

For example, the enthalpy of formation of cyclopropane is +53.6 kJ mol-1, whereas 3 the hypothetical enthalpy of formation based on three "normal" methylene groups, from 4 acyclic models, is -62 kJ mol-1. On this basis cyclopropane is destabilized by ca. 115 kJ 5 mol–1 of strain energy. 6

See molecular mechanics. 7 [3] 8

9 structural isomers 10 discouraged term for constitutional isomers. 11 12 subjacent orbital 13 Next-to-Highest Occupied Molecular Orbital (NHOMO, also called HOMO–1). 14

Note: Subjacent and superjacent orbitals sometimes play an important role in the 15 interpretation of molecular interactions according to the frontier orbital approach. 16

See [381]. 17 [3] 18

19 substituent 20 Any atom or group of bonded atoms that can be considered to have replaced a 21 hydrogen atom (or two hydrogen atoms in the special case of bivalent groups) in a 22 parent molecular entity (real or hypothetical). 23

[3] 24 25 substitution 26 Chemical reaction, elementary or stepwise, of the form A–B + C → A–C + B, in which 27 one atom or group in a molecular entity is replaced by another atom or group. 28

Examples 29 CH3Cl + HO– →! CH3OH + Cl– 30

C6H6 + NO2+!→!!$6H5NO2 + H+ 31

Note: A substitution reaction can be distinguished as an electrophilic substitution or 32 a nucleophilic substitution, depending on the nature of the reactant that is considered 33 to react with the substrate. 34

rev[3] 35 36 substrate 37 Chemical species, the reaction of which with some other chemical reagent is under 38 observation (e.g., a compound that is transformed under the influence of a catalyst). 39

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Note: The term should be used with care. Either the context or a specific statement 1 should always make it clear which chemical species in a reaction is regarded as the 2 substrate. 3

See also transformation. 4 [3] 5

6 successor complex 7 Chemical species formed by the transfer of an electron or of a hydrogen (atom or ion) 8 from a donor D to an acceptor A after these species have diffused together to form the 9 precursor or encounter complex: 10

11 rev[3] 12 13

suicide inhibition 14 See mechanism-based inhibition. 15 [3] 16

17 superacid 18 Medium having a high acidity, generally greater than that of 100 % sulfuric acid. The 19 common superacids are made by dissolving a powerful Lewis acid (e.g., SbF5) in a 20 suitable Brønsted acid, such as HF or HSO3F. 21

Note 1: An equimolar mixture of HSO3F and SbF5 is known by the trade name Magic 22 Acid. 23

Note 2: An uncharged gas-phase substance having an endothermicity (enthalpy) of 24 deprotonation (dehydronation) lower than that of H2SO4 is also called a superacid. 25 Nevertheless such a superacid is much less acidic in the gas phase than similar cationic 26 acids. 27

See [18,382,383,384]. 28 See acidity, superbase. 29 rev[3] 30

31 superbase 32 Compound having a very high basicity. 33

Examples include amide bases such as LDA, potassium tert-butoxide + 34 organolithium, some phosphazenes. 35

See superacid. 36 See [385]. 37 rev[3] 38

39

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superjacent orbital 1 Second Lowest Unoccupied Molecular Orbital (SLUMO). 2

Note: Subjacent and superjacent orbitals sometimes play an important role in the 3 interpretation of molecular interactions according to the frontier orbital approach. 4

See [381]. 5 6 suprafacial 7

See antarafacial. 8 [3] 9

10 supramolecular 11 Description of a system of two or more molecular entities that are held together and 12 organized by means of intermolecular (noncovalent) binding interactions. 13

See [386]. 14 [3] 15

16 Swain-Lupton equation 17 Dual-parameter approach to the correlation analysis of substituent effects, which 18 involves a field constant (F) and a resonance constant (R). 19 20

lg(kX/kH) = fFX + rRX 21 22

See [191,387]. 23 Note 1: The original treatment was modified later. 24 Note 2: The procedure has often been applied, but also often criticized. 25 See [388,389,390,391,392]. 26 [3] 27

28 Swain-Scott equation 29 Linear Gibbs-energy relation (linear free-energy relation) of the form 30 31

lg(k/k0) = sn 32 33 applied to the variation of reactivity of a given electrophilic substrate towards a series 34 of nucleophilic reagents, where k0 is a rate constant for reaction with water, k is the 35 corresponding rate constant for reaction with any other nucleophilic reagent, n is a 36 measure of the nucleophilicity of the reagent (n = 0.0 for water) and s is a measure of 37 the sensitivity of the substrate to the nucleophilicity of the reagent (s = 1.0 for CH3Br). 38

See [192]. 39 See also Mayr-Patz equation, Ritchie equation. 40

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[3] 1 2 symproportionation 3 comproportionation 4

[3] 5 6 syn 7

See anti. 8 See [11]. 9 [3] 10

11 synartetic acceleration 12

See neighbouring group participation. 13 [3] 14

15 synchronization 16

See principle of nonperfect synchronization. 17 rev[3] 18

19 synchronous 20 Feature of a concerted process in which all the changes (generally bond rupture and 21 bond formation) have progressed to the same extent at the transition state. 22

Note 1: A synchronous reaction is distinguished from (1) a concerted reaction, which 23 takes place in a single kinetic step without being synchronous, (2) a reaction where 24 some of the changes in bonding take place earlier, followed by the rest, and (3) a two-25 step reaction, which takes place in two kinetically distinct steps, via a stable 26 intermediate 27

Note 2: The progress of the bonding changes (or other primitive changes) is not 28 defined quantitatively in terms of a single parameter applicable to different bonds. The 29 concept therefore does not admit an exact definition except in the case of concerted 30 processes involving changes in two identical bonds. If the bonds are not identical, the 31 process should simply be described as concerted. 32

See [393,394,395]. For an index of synchronicity see [396]. 33 See also imbalance. 34 rev[3] 35

36 s, p 37 Symmetry designations that distinguish molecular orbitals as being symmetric (s) or 38 antisymmetric (p) with respect to a defining plane containing at least one atom. 39

Note 1: In practice the terms are used both in this rigorous sense (for orbitals 40

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encompassing the entire molecule) and for localized two-centre orbitals or bonds. In 1 the case of localized two-centre bonds, a p bond has a nodal plane that includes the 2 internuclear bond axis, whereas a sbond has no such nodal plane. (A d bond in 3 organometallic or inorganic chemical species has two nodes.) Radicals are classified 4 by analogy into s and p radicals. 5

Note 2: Such two-centre orbitals may take part in molecular orbitals of s or p 6 symmetry. For example, the methyl group in propene contains three C–H bonds, each 7 of which is of local s symmetry (i.e., without a nodal plane including the internuclear 8 axis), but these three s bonds can in turn be combined to form a set of group orbitals 9 one of which has p symmetry with respect to the principal molecular plane and can 10 accordingly interact with the two-centre orbital of p symmetry (p bond) of the double-11 bonded carbon atoms, to form a molecular orbital of p symmetry. Such an interaction 12 between the CH3 group and the double bond is an example of hyperconjugation. This 13 cannot rigorously be described as s-p conjugation since s and p here refer to different 14 defining planes, and interaction between orbitals of different symmetries (with respect 15 to the same defining plane) is forbidden. 16

Note 3: Conjugation between a p system and a lone pair of (for example) an ether 17 oxygen involves a lone pair of p symmetry with respect to the defining plane. It is 18 incorrect to consider this as an interaction between the p system and one of two 19 identical sp3-hybrid lone pairs, which are neither s nor p. 20

Note 4: The two lone pairs on a carbonyl oxygen are properly classified as s or p 21 with respect to the plane perpendicular to the molecular plane (dotted line perpendicular 22 to the plane of the page), rather than as two sp2-hybridized lone pairs. This distinction 23 readily accounts for the facts that there are two different lone-pair ionization energies 24 and two different n-p* excited states. 25

26

27 28 29 30 31

See also [397]. 32 33 s-adduct 34 Product formed by the attachment of an electrophilic or nucleophilic entering group or 35 of a radical to a ring carbon of an aromatic species so that a new s bond is formed and 36 the original conjugation is disrupted. 37

Note 1: This has generally been called a s complex, or a Wheland complex from 38

R

RO not

R

RO

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electrophilic addition, but adduct is more appropriate. 1 Note 2: The term may also be used for analogous adducts to p systems. 2 See also Meisenheimer adduct. 3 rev[3] 4

5 s-constant 6 Substituent constant for meta- and para-substituents in benzene derivatives as defined 7 by Hammett on the basis of the ionization constant of a substituted benzoic acid, i.e., 8 lg(KaX/KaH), where KaX is the ionization constant (acid-dissociation constant) of a m- or 9 p-substituted benzoic acid and KaH that of benzoic acid itself. 10

Note 1: A large positive s-value implies high electron-withdrawing power by an 11 inductive and/or resonance effect, relative to H; a large negative s-value implies high 12 electron-releasing power relative to H. 13

Note 2: The term is also used as a collective description for related electronic 14 substituent constants based on other standard reaction series, of which, s+,s– and so 15 are typical; also for constants which represent dissected electronic effects, such as sI 16 and sR. For example,s– (sigma-minus) constants are defined on the basis of the 17 ionization constants of para-substituted phenols, where such substituents as nitro show 18 enhanced electron-withdrawing power. 19

See [124,125,126,191,226,398]. 20 See also Hammett equation, r-value, Taft equation. 21 [3] 22

23 Taft equation 24 Linear free-energy relation (linear Gibbs-energy relation) involving the polar substituent 25 constant s* and the steric substituent constant Es, as derived from reactivities of 26 aliphatic esters 27 28

lg {k} = lg {k0} + ρ*σ* + d Es 29 30 The argument of the lg function should be of dimension 1. Thus, the reduced rate 31 constants should be used, i.e., the rate coefficient divided by its units: {k} = k/[k] and 32 {k0} = k0/[k0]. 33

Note: The standard reaction (k0) is the hydrolysis of methyl acetate, whereby Es is 34 evaluated from the rate of acid-catalyzed hydrolysis, relative to that of methyl acetate, 35 and s* is evaluated from the ratio of the rates of base- and acid-catalyzed hydrolysis. 36

See [172,380,399,400]. 37 See also Hammett equation, r-value, s-constant. 38 rev[3] 39

40

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tautomerization 1 Rapid isomerization of the general form 2

G–X–Y=Z ⇌ X=Y–Z–G 3 where the isomers (called tautomers) are readily interconvertible. 4

Note 1: The atoms of the groups X,Y, Z are typically any of C, H, N, O, or S, and G 5 is a group that becomes an electrofuge or nucleofuge during isomerization. 6

Note 2: The commonest case, when the electrofuge is H+, is also known as a 7 prototropic rearrangement. 8

Examples: 9 10

11 12

Note 3: The group Y may itself be a three-atom (or five-atom) chain extending the 13 conjugation, as in 14 15

16 17

Note 4: Ring-chain tautomerization is the case where addition across a double bond 18 leads to ring formation, as in 19 20

21 Note 5: Valence tautomerization is the case of rapid isomerization involving the 22

formation and rupture of single and/or double bonds, without migration of atoms: for 23 example 24

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1 See [346,401]. 2 See also ambident, fluxional, sigmatropic rearrangement, valence tautomerization. 3

rev[3] 4 5

tautomers 6 Constitutional isomers that can interconvert more or less rapidly, often by migration of 7 a hydron. 8

See tautomerization. 9 10 tele-substitution 11 Substitution reaction in which the entering group takes up a position more than one 12 atom away from the atom to which the leaving group was attached. 13

Example 14 15

16 See also cine-substitution. 17 See [402,403]. 18 rev[3] 19

20 termination 21 Step(s) in a chain reaction in which reactive intermediates are destroyed or rendered 22 inactive, thus ending the chain. 23

[3] 24 25 tetrahedral intermediate 26 Reaction intermediate in which the bond arrangement around an initially double-bonded 27 carbon atom (typically a carbonyl carbon) has been transformed from tricoordinate to 28 tetracoordinate (with a coordination number of 4). 29

Example: 30

31 rev[3] 32

33 34

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thermodynamic control (of product composition) 1 equilibrium control 2 Conditions (including reaction times) that lead to reaction products in a proportion 3 specified by the equilibrium constant for their interconversion. 4

See also kinetic control. 5 rev[3] 6

7 thermolysis 8 Uncatalysed cleavage of one or more covalent bonds resulting from exposure of a 9 molecular entity to a raised temperature, or a process of which such cleavage is an 10 essential part. 11

See also pyrolysis. 12 [3] 13

14 through-conjugation 15 Phenomenon whereby electrons can be delocalized from any of three (or more) groups 16 to any other. 17

Example: p-XC6H4Y, where an electron pair can be delocalized from electron-18 donating X not only to ring carbons but also to electron-withdrawing Y. 19

Note 1: This may be contrasted with cross-conjugation. 20 Note 2: In Hammett-type correlations (linear Gibbs-energy relationships) this 21

situation can lead to exalted substituent constants s+ or s–, as in solvolysis of p-22 CH3OC6H4C(CH3)2Cl or acidity of p-nitrophenol, respectively. 23 24 TICT 25

See twisted intramolecular charge transfer. 26 27 torquoselectivity 28 Preference for inward or outward rotation of substituents in conrotatory or disrotatory 29 electrocyclic ring-opening and ring-closing reactions, often owing to a preference for 30 electron donors (especially including large groups) to rotate outward and acceptors to 31 rotate inward. 32

See [404,405]. 33

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1 rev[3] 2

3 transferability 4 Assumption that a chemical property associated with an atom or group of atoms in a 5 molecule will have a similar (but not identical) value in other circumstances. 6

Examples: equilibrium bond length, bond force constant, NMR chemical shift. 7 [3] 8

9 transformation 10 Conversion of a substrate into a particular product, irrespective of the specific reagents 11 or mechanisms involved. 12

Example: transformation of aniline (C6H5NH2) into N-phenylacetamide 13 (C6H5NHCOCH3), which may be effected with acetyl chloride or acetic anhydride or 14 ketene. 15

Note: A transformation is distinct from a reaction, the full description of which would 16 state or imply all the reactants and all the products. 17

See [406]. 18 [3] 19

20 transient (chemical) species 21 Short-lived reaction intermediate. 22

Note 1: Transiency can be defined only in relation to a time scale fixed by the 23 experimental conditions and by the limitations of the technique employed in the 24 detection of the intermediate. The term is a relative one. 25

CH3

CH3

H3C

H3C

CH3

CH3

H3C

H3C

H3C

H3C

CH3

CH3not

CH=OCH=O

CH=Onot

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Note 2: Transient species are sometimes also said to be metastable. However, this 1 latter term should be avoided, because it relates a thermodynamic term to a kinetic 2 property, although most transients are also thermodynamically unstable with respect to 3 reactants and products. 4

See also persistent. 5 [3] 6

7 transition dipole moment 8 Vector quantity describing the oscillating electronic moment induced by an 9 electromagnetic wave, given by an integral involving the dipole moment operator m and 10 the ground- and excited-state wavefunctions: 11 12

M = ò Yexc m Ygnd dt 13 14

Note: The magnitude of this quantity describes the allowedness of an electronic 15 transition. It can often be separated into an electronic factor and a nuclear-overlap factor 16 known as the Franck-Condon factor. 17

See [9]. 18 19 transition state 20 State of a molecular system from which there are equal probabilities of evolving toward 21 states of lower energy, generally considered as reactants and products of an 22 elementary reaction. 23

Note 1: The transition state corresponds to the maximum along the minimum-Gibbs-24 energy path connecting reactants and products. 25

Note 2: The transition state can be considered to be a chemical species of transient 26 existence. 27

Note 3: The term transition-state structure refers to a structure inferred from kinetic 28 and stereochemical investigations; it does not necessarily coincide with a transition 29 state or with a transition structure, which corresponds to a saddle point on a potential-30 energy surface, although it may represent an average structure. 31

Note 4: The assembly of atoms at the transition state has also been called an 32 activated complex, although it is not a complex according to the definition in this 33 Glossary. 34

Note 5: There are also reactions, such as the gas-phase colligation of simple radicals 35 or the reactions of some reactive intermediates in solution, that do not require activation 36 and do not involve a transition state. 37

Note 6: Ultrafast spectroscopy permits observation of transition states in some 38 special cases. 39

See [349,407]. 40

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See also Gibbs energy of activation, potential-energy profile, reaction coordinate, 1 transition structure. 2

rev[3] 3 4 transition-state analogue 5 Species designed to mimic the geometry and electron density of the transition state of 6 a reaction, usually enzymatic. 7

Note: A transition-state analogue is usually not a substrate for the enzyme, but rather 8 an inhibitor. 9

rev[3] 10 11 transition structure 12 Molecular entity corresponding to a saddle point on a potential-energy surface, with one 13 negative force constant and its associated imaginary frequency. 14

Note 1: Whereas the transition state is not a specific molecular structure, but a set 15 of structures between reactants and products, a transition structure is one member of 16 that set with a specific geometry and energy. 17

Note 2: Although the saddle point coincides with the potential-energy maximum 18 along a minimum-energy reaction path, it does not necessarily coincide with the 19 maximum of Gibbs energy for an ensemble of chemical species. 20

Note 3: The term transition-state structure is not a synonym for transition structure. 21 See [349,408]. 22 See also activated complex, transition state. 23 rev[3] 24

25 transition vector 26 Normal mode of vibration of a transition structure corresponding to the single imaginary 27 frequency and tangent to the intrinsic reaction coordinate at the saddle point. 28 Sometimes called the reaction-coordinate vibrational mode. 29

Note: Infinitesimal motion along the transition vector in the two opposite senses 30 determines the initial direction leading either toward reactants or toward products. 31

Note 2: The term "transition coordinate" was used in the 1994 Glossary of Terms in 32 Physical Organic Chemistry in the sense defined here for transition vector, but 33 apparently this usage was unique in the literature at that time and has not been 34 generally adopted since. 35

See [349,409]. 36 See reaction coordinate, transition state. 37

38 39 40

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transport control 1 See encounter-controlled rate, microscopic diffusion control. 2 [3] 3

4 trapping 5 Interception of a reactive molecule or reaction intermediate so that it is removed from 6 the system or converted into a more stable form for study or identification. 7

See also scavenger. 8 [3] 9

10 triplet state 11 State having a total electron spin quantum number of 1. 12

See [9]. 13 14 tunnelling 15 Quantum-mechanical phenomenon by which a particle or a set of particles penetrates 16 a barrier on its potential-energy surface without having the energy required to surmount 17 that barrier. 18

Note 1: In consequence of Heisenberg's Uncertainty Principle, a molecular entity has 19 a nonzero probability of adopting a geometry that is classically forbidden because it 20 corresponds to a potential energy greater than the total energy. 21

Note 2: Tunnelling is often considered to be a correction to an (over)simplified 22 version of transition-state theory. 23

Note 3: Because the rate of tunnelling increases with decreasing mass, it is 24 significant in the context of isotope effects, especially of hydrogen isotopes. 25

See [410,411]. 26 rev[3] 27

28 twisted intramolecular charge transfer (TICT) 29 Feature of an excited electronic state formed by intramolecular electron transfer from 30 an electron donor (D) to an electron acceptor (A), where interaction between electron 31 and hole is restricted because D+ and A– are perpendicular to each other. 32

See [412]. 33 34 umpolung 35 Process by which the nucleophilic or electrophilic property of a functional group is 36 reversed. 37

Note: Umpolung is often achieved by temporary exchange of heteroatoms N or O by 38 others, such as P, S, or Se, as in the conversion of electrophilic RCH=O to RCH(SR')2 39

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and then with base to nucleophilic RC(SR')2–. Also the transformation of a haloalkane 1 RX into a Grignard reagent RMgCl is an umpolung. 2

See [413]. 3 rev[3] 4

5 unimolecular 6 Feature of a reaction in which only one molecular entity is involved. 7

See molecularity. 8 rev[3] 9

10 unreactive 11 Failing to react with a specified chemical species under specified conditions. 12

Note: The term should not be used in place of stable, which refers to a 13 thermodynamic property, since a relatively more stable species may nevertheless be 14 more reactive than some reference species towards a given reaction partner. 15

[3] 16 17 unstable 18 Opposite of stable, i.e., the chemical species concerned has a higher molar Gibbs 19 energy than some assumed standard. 20

Note: The term should not be used in place of reactive or transient, although more 21 reactive or transient species are frequently also more unstable. 22

rev[3] 23 24 upfield 25 superseded but still widely used to mean shielded. 26

See chemical shift. 27 [3] 28

29 valence 30 Maximum number of single bonds that can be commonly formed by an atom or ion of 31 the element under consideration. 32

Note: Often there is a most common maximum for a given element, and atoms in 33 compounds where this number is exceeded, such as pentacoordinate carbocations 34 (“carbonium ions”) and iodine(III) compounds, are called hypervalent. 35

rev[3] 36 37 valence isomer 38 Constitutional isomer related to another by pericyclic reaction. 39

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Examples: Dewar benzene, prismane, and benzvalene are valence isomers of 1 benzene. 2

Note: Valence isomers are separable, as distinguished from valence tautomers, 3 which interconvert rapidly. See valence tautomerization. 4

rev[3] 5 6 valence tautomerization 7 Rapid isomerization involving the formation and rupture of single and/or double bonds, 8 without migration of atoms. 9

Example: 10

11 See tautomerization. 12 rev[3] 13

14 van der Waals forces 15 Attractive or repulsive forces between molecular entities (or between groups within the 16 same molecular entity) other than those due to bond formation or to the electrostatic 17 interaction of ions or of ionic groups with one another or with neutral molecules. 18

Note 1: The term includes dipole-dipole, dipole-induced dipole, and London 19 (instantaneous induced dipole-induced dipole) forces, as well as quadrupolar forces. 20

Note 2: The term is sometimes used loosely for the totality of nonspecific attractive 21 or repulsive intermolecular forces. 22

Note 3: In the context of molecular mechanics, van der Waals forces correspond only 23 to the London dispersion forces plus the Pauli repulsion forces. Interactions due to the 24 average charge distribution (and not to fluctuations around the average or to induced 25 dipoles), including dipole-dipole, ion-ion and ion-dipole forces, are called Coulombic 26 forces. 27

See [290,414]. 28 See also dipole-dipole interaction, dipole-induced dipole forces, London forces. 29 rev[3] 30

31 volume of activation, ∆‡V 32 Quantity derived from the pressure dependence of the rate constant of a reaction, 33 defined by the equation 34 35

∆‡V = -RT [∂ln(𝑘/[𝑘])/∂𝑝]T 36 37

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provided that rate constants of all reactions (except first-order reactions) are expressed 1 in pressure-independent concentration units, such as mol dm–3 at a fixed temperature 2 and pressure. The argument in the lg function should be of dimension 1. Thus, the rate 3 constant should be divided by its units, [k]. 4

Note: The volume of activation is interpreted as the difference between the partial 5 molar volume ‡V of the transition state and the sums of the partial molar volumes of the 6 reactants at the same temperature and pressure, i.e., 7 8

∆‡V = ‡V – S(rVR) 9 10 where r is the order in the reactant R and VR its partial molar volume. 11

See [13]. 12 See order of reaction. 13 rev[3] 14

15 water/octanol partition coefficient (partition ratio) 16

See octanol-water partition ratio 17 18 wavefunction 19 A mathematical expression whose form resembles the wave equations of physics, 20 supposed to contain all the information associated with a particular atomic or molecular 21 system. In particular, a solution of the Schrödinger wave equation, Hy = Ey, as an 22 eigenfunction y of the hamiltonian operator H, which involves the electronic and/or 23 nuclear coordinates. 24

Note 1: The wavefunction contains all the information describing an atomic or 25 molecular system that is consistent with the Heisenberg Uncertainty Principle. 26

Note 2: When a wavefunction is operated on by certain quantum-mechanical 27 operators, a theoretical evaluation of physical and chemical observables for that system 28 (the most important one being energy) can be carried out. 29

See [8,12]. 30 31 Wheland intermediate 32

See Meisenheimer adduct, s-adduct. 33 [3] 34

35 Woodward-Hoffmann rules 36

See orbital symmetry. 37 38 39 40

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ylide 1 Chemical species that can be produced by loss of a hydron from an atom directly 2 attached to the central atom of an onium ion. 3

Example: 4 Ph3P+–CHRR’ → [Ph3P+–C–RR’ ↔ Ph3P=CRR’] + H+ 5

See [40]. 6 [3] 7

8 Yukawa-Tsuno equation 9 Multiparameter extension of the Hammett equation to quantify the role of enhanced 10 resonance effects on the reactivity of para–substituted benzene derivatives, 11 12

lg {k} = lg {ko} + r[so + r(s+ –so)] 13 or lg {k} = lg {ko} + r[σo + r(s– –so)] 14 15 where the parameter r expresses the enhancement and where so is a substituent 16 constant based on reactivities of phenylacetic acids and similar substrates, where 17 resonance interaction is weak or absent. The argument in the lg function should be of 18 dimension 1. Thus, reduced rate constants should be used {k}= k/[k] and {ko} = ko/[ko]. 19

See [398,415,416]. 20 See also dual substituent-parameter equation, r-value, s-constant, through-21

conjugation. 22 rev[3] 23

24 Zaitsev rule 25

See Saytzeff rule. 26 [3] 27

28 zero-point energy 29 Extent, in consequence of Heisenberg's Uncertainty Principle, by which a particle or a 30 set of particles has an energy greater than that of the minimum on the potential-energy 31 surface. 32

Note 1: Because of zero-point energy a molecular entity has a nonzero probability of 33 adopting a geometry whose energy is greater than that of the energy minimum. 34

Note 2: A molecular entity with zero-point energy may even adopt a geometry with 35 a potential energy greater than its total energy, a possibility that permits tunnelling. 36

Note 3: Because the magnitude of zero-point energy increases with decreasing 37 mass, it is significant in the context of isotope effects, especially of hydrogen isotopes. 38 39 Zucker-Hammett hypothesis 40

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Assumption that if lg {k1} (= k1/[k1], reduced pseudo-first-order rate constant of an acid-1 catalyzed reaction) is linear in Ho (Hammett acidity function), then water is not involved 2 in the transition state of the rate-controlling step, whereas if lg {k1} is linear in lg {[H+]} 3 then water is involved. The argument in the lg function should be of dimension 1. Thus, 4 reduced concentration = {[H+]} should be used, i.e., concentration of protons divided by 5 its units. 6

Note: This has been shown to be an overinterpretation. 7 See [22,417]. 8 See also Bunnett-Olsen equation, Cox-Yates equation. 9 rev[3] 10

11 Z-value 12 Quantitative measure of solvent polarity based on the UV-vis spectrum of 1-ethyl-4-13 (methoxycarbonyl)pyridinium iodide. 14


18 zwitterion 19 Highly dipolar, net uncharged (neutral) molecule having full electrical charges of 20 opposite sign, which may be delocalized within parts of the molecule but for which no 21 uncharged canonical resonance structure can be written. 22

Examples: glycine (H3N+–CH2–CO2!), betaine (Me3N+–CH2–CO2!). 23

Note 1: Sometimes also referred to as inner salts or ampholytes. 24 Note 2: Mesoionic compounds, such as sydnones, in which both positive and 25

negative charge are delocalized, are sometimes considered as zwitterions, but species 26 with a localized nonzero formal charge, such as a nitrone, CH3CH=N+(–O–)CH3, are 27 not. 28

See [418]. 29 rev[3] 30

31

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